Car battery packs are made up internally of standard size rechargeable batteries. Tesla, over the years has used, 18650, 2170 and 4680 batteries. The numbers relate directly to their size but these size batteries are all available on Amazon etc. There's a lot of them in a pack and they're surrounded by coolant so you'd either have to be insane or very skilled to want to change a battery out as the voltage and amperage will definitely kill you if you make a mistake. I think the packs for cars are made up from smaller standardised packs (or modules) that make it easier (and cheaper) to build the complete pack.
Most modern battery packs seem to last very well. There are plenty of Tesla's etc running fine with more than 200,000 miles on them. Used Tesla's with high mileages are seen as a bit of a bargain over here as they're almost indistinguishable from new ones but only 20% of the price. Obviously you still have that association with Elon though.