How does After Market car stuff work???

Hello! I'm absolutely terrible with cars and I find them extremely overwhelming but I have to buy one. I was looking at Toyotas and Hondas at my local CarMax because they're really solid, long lasting cars. I want apple car play but its not been in a lot of the cars I've seen at my budget.

The salesperson mentioned After Market and that was my very first time hearing about it. Apparently they can install that in an older car that dosnt originally have it? How does that work? Where do I find After Market stuff? I have a million questions

I literally know nothing and am very confused and overwhelmed with all car related things. Please help