A little late to this, but, here's my experience. I have to say, my initial understanding of how the system worked was completely wrong. Took a while to work out what was going on in different modes.
This is specifically about the 330e (and that is a 5 year old model). Other BWM models are or maybe configured differently and will operate differently. There is a lot to unpack here....
The simple way to view things is - The 330e ICE and electric motor are in line on the powertrain. The electric motor is part of the gearbox. The gearbox itself works pretty much just the same in EV mode as in ICE mode. The ICE and HV system work in concert with each other, switching modes and trading energy as demand calls.
The standard Auto mode will elect to use the HV battery until depleted, or, speed exceeds 50mph, or, you call for more power than the battery system alone can supply (80 BHP) - ie. hard acceleration or going up a or even starting on steep hill.
Even at high speed, the battery system can and will augment the ICE under low torque demand - flat terrain, even speed, etc - you will see mpg get better and power flow from the battery, even maybe only for a few seconds at a time. The two systems constantly work together. When you call for a little more power (eg just going up a slight incline), some extra EV power is added to the ICE and, when available, is power bled off by the electric motor acting as generator to recharge the battery (if the battery is depleted below its baseline level). That means the ICE operates more a little more efficiently using less fuel itself when it isn't needed and can be augmented by the HV system, but, then uses just a little more when it is in demand and the battery level needs to be bought up above complete discharge. The system tries to maintain a baseline distance of 1 to 2 miles electric coverage as far as it can by bleeding that energy from the ICE. That additional fuel usage to do that is less noticeable and is done in a higher power band in the ICE. You will often see no range or zero miles in the battery which then increases to 1m (which is closer to 2 miles in reality). The ICE recharges the HV battery when needed.
If you look on the app, the distance is always one mile greater than the in car display - that shows the rounded down integer number of miles available - that played around with my head for a while!!!.
So, under normal conditions when the HV battery has been used up, a little charge is fed back when some is available without massively increasing fuel consumption, and then the EV system takes over again under low torque demand. That process keeps recycling. Just like a standard HEV.
Energy is recovered to the battery when braking and coasting (by the electric motor on the gearbox) although the physical brakes will cut in under harder braking and the final stop. The balance of recharging and recovery rates also changes between standard and Eco modes. Perversely there seems to be less recovery in ECO mode when you take your foot off the accelerator and coasting - I presume that is if you are more careful when slowing down, you can lift off earlier than in Auto mode and coast to a stop over a longer distance - the effect of energy recovery braking is less. Comfort mode slows you down more quickly (without using the brakes) when coasting because energy is recovered more aggressively.
Eco mode reduces the profile of the accelerator making it more gentle than Comfort or Sport modes.
Any passenger compartment heating or cooling demand really sucks power out the HV battery. This effect is more pronounced when the HV battery is cold itself so range is already reduced. Time preconditioning can help if you can. But, the HV battery heater isn't much help when below zero degrees (Centigrade) - that heater is only available with preconditioning for a time departure. It doesn't have time to ward the battery when you switch on preconditioning manually. It takes an hour to two to heat the HV battery it seems.
Eco mode I also suspect reduces the feed to the electric steering motor - That is a big user of power. Inputting large and aggressive steering inputs will also drain the HV battery more quickly.
Have found the satnav has little effect on the switching from EV to ICE mode - I suspect that while it has some knowledge about the type and speed of roads in your planned journey, it has no knowledge of terrain.
Terrain makes a big difference too. I have a 4 mile uphill journey to get to the motorway. That just sucks energy from the HV battery. On the basis that I will often then be on the motorway often for 50-100 miles, it seems to be beneficial to put the system into save mode when setting out, which means the battery energy is retained for later and the ICE is already well warmed up before hitting the motorway. The thought of dropping into high load at speed on a stone cold engine doesn't sound a kind way to treat the ICE.
There is an awful lot of complexity in all this.
All that said, the range of my battery system has plummeted over the last couple of years. I could originally and easily get 25+ miles on a charge. That then dropped initially to approx 17 miles, then 11 miles and then recently plummeted to 5-6 miles recently. The dealer has, despite a number of trips for diagnosis, only just admitted that two of the five battery modules in the HV battery are faulty - that was shown after many Drivetrain warnings and even red alerts (ie STOP!). They are being replaced today under the 6 year manufacturer battery warranty.
Despite the problem of much reduced range and consequential inefficient charging due to the battery fault, the car has still returned 55+mpg at an average speed of over 35mph over 30k miles. That I think is a high return over a long distance at a mix of speeds, both slower urban driving and high motorway speeds. I think this shows how the electronics moves power around using and saving battery power as demand calls. I do probably think far more than I should and is healthy as to which is the best strategy for each journey. I am usually careful using my right foot, but, at times, I am no angel. That said - I am fortunate and have a Z4 for more 'creative' driving. That average mpg has dropped from 65mpg over three years due to the fault as more ICE miles have been done due to the lower electric range.
If you want to drive without any understanding of how the system works, you can (and should). Plug it in and recharge when you can (and make the most of overnight electricity rates if you can). Leave it in Auto and Comfort or switch to Eco mode if you can. The car simply uses up any available charge and then reverts to working like a standard HEV unless at some point you trip one of the conditions to cut to ICE. If you are prepared to think a little more carefully, you can extract a great deal more EV range when it is best to be in that mode.
The cost of charging is still less than petrol especially on a cold engine. Most local journeys are often just a few miles. A cold ICE is operating at 20-25mpg in those circumstances. Use electric for those when you can. Even plug in and recharge at day rates if you can - it is still often cheaper than using a cold ICE.
I do however expect, that in years to come, AI will be used to learn over time how and where you drive to maximise the EV and ICE characteristics. It won't be perfect, but, it will help.
Just another observation - Winter tyres cost about 5mpg. That was a surprise.
As I said, a lot to unpack. Hope it helps. Apologies if it doesn't.