I’m just going through the process myself and I agree there’s a lot to get your head around. All the info can be found on the website below
But in a nutshell:
You exchange your high rate mobility for a car - you keep the care part. Once you’ve got your car it is all sorted out and you don’t see that part of the money again - it goes directly to pay for the car.
Most leases seem to be for 3 years - at the end of that time you order a new car and give the old one back.
There are various makes and models available in petrol/diesel/automatic versions - you just need to choose what would be suitable for you (and choose the colour of course) All the models are detailed on the website but most garages have a motability expert who knows all the finer details.
Some cars need an advanced payment (presumably as they’re a little more expensive than the others) There are plenty that don’t require any extra payment apart from your DLA.
You just need to take your DLA proof and driving licence to your chosen garage and they will sort it all out.
As far as adaptations go, if you think you may need them Motability will refer you to the assessment centre where they will see what is best for you. I’m going to Newcastle on Friday, it’s 50 miles away but I thought it would be agood idea. If I’d self -referred it would have cost me £25.
I think that covers most things but the website link explains things in detail