You can, however there's three problems.

Firstly from an aesthetic point of view they're hideous, you'll end up having to buy a plastic piece of crap to hold it in and aftermarket stereos are just not great, it won't be flush to the rest of the dash.

Secondly depending on your car a bunch of stuff that used to work with your car (forward/reverse sensors) might not show up any more.

Thirdly stalk adaptors costs a fortune.

You really want a car that has got that experience sorted and after driving a modern VW, I'd say a VW or Audi is probably the way to go. They'll have Carplay/Auto/Mirror and that's what you want. The experience of an aftermarket stereo is just not great.

Some cars you can. If they have a standard 1- or 2-DIN radio, or an adapter for same, then yeah you can retrofit a CarPlay-capable radio.

But many newer cars have totally custom, integrated audio systems and retrofitting is either extremely expensive or impossible.

Normal cars, yes. A Tesla… I don't think so. The infotainment center in the Tesla isn't in the DIN form factor, and it's tightly integrated with other vehicle functionality like HVAC controls, cruise control / Autopilot, battery status, etc. There's no way of replicating those features with an aftermarket stereo.

On vehicles in which you can change the infotainment yeah. But this doesnt really go well for the tesla.

In addition, to my knowledge older Volkswagen AG vehicles are ripe for replacing the infotainment with a newer, carplay enabled system, but on newer models it seems that it is rather locked down and not as easy a solution as you suggest.

True, but system integration is a mixed bag when you do. When it is built in originally, you are assured it will work w/ steering wheel controls, etc.

On the flipside, some manufacturers impose limitations on what you can do when driving, which is annoying, so after market might get around some of that?

I haven't seen a car with a standard DIN1/2 amplifier in a very long time.

My last car, the "stereo" was just a control board with a screen and some buttons. The head-unit was actually under the seat. The space directly behind the stereo controller was air ducts and a center speaker.

Yes, I have a Miata, which I modified for this purpose. I really like my CarPlay setup.