... so when you're asking stuff like model names, you mean specific producers or just general types of firearm? There's big names for the former (Remington, Smith and Wesson, etc.), but to be honest I don't think most of us really pay
that much attention to it. More enthusiastic firearm owners definitely do, but your hypothetical common firearm owner, less so, and even less than that beyond the point of purchase. S'kinda' like major household electrical appliances, so far as that goes. If you actually want a list googling (or whatever) "major US firearm manufacturers" is pretty easy, and will get you the information you want.
If you're just talking types of guns, shotguns, .22s, revolvers, and just general pistols, are probably the most ubiquitous/normal, iirc, though it'll vary by area a fair bit (hand guns more common in cities, shotguns/hunting rifles/"varmint guns" outside). You can indeed roll up to walmart and buy certain firearms (mostly longarms and shotguns, iirc; I don't pay too much attention when I walk by the sporting goods section, tbh) and bullets. There
are background checks involved (or should be, anyway, which isn't a guarantee) for buying firearms, but they're not exactly difficult to pass, most of the time, though I forget how long they usually take... at least a day or two, but more than that I don't recall. Most likely place is just from a gun shop, though, which are pretty common in a number of areas and do require a check (if they want to stay in business anyway, providing the area's not too corrupt).
Fastest way is via gun shows or local bartering (buying from friends, etc.), and how much vetting the former requires varies by region. The latter does tend to have some laws on the books regarding who you can and can't and whether/how it needs to be reported, but enforcement on that end is... spotty. The BATF is frankly more or less crippled, so a lot of the smaller stuff just doesn't have the resources or legal freedom to really police.
Ammunition's usually sold wherever you can get firearms, obviously enough, and usually packaged in boxes, either cardboard or plastic. Forget exactly how many is common for a box but I
want to say 1-200 bullet boxes of .22 ammo is fairly normal? Or was, last I was around someone buying. It's been a long, long time since I've actually bought ammunition. Though now that I think about it, let me get up for a second -- family's got ammunition sitting around the house... and the answer looks to be 20 to 50, rifle and pistol respectively, apparently. Cardboard boxes. Depends on the caliber, basically. You can definitely get larger ones, though, and it's not difficult to go out and buy/order a steel drum (like, those things stereotyped as being used by hobos to burn things in to keep warm, iirc holding multiple thousand rounds) full. Name wise most of them are made more or less by the same folks that make the guns, iirc.
Here's a nice list, though.Care... honestly, I've forgotten most of it. I do own a rifle, but don't handle the maintenance and haven't fired it in over a decade to boot so *shrugs* If I actually needed to do some touch up I'd just google info, the process (up to and including videos) is silly easy to find. I do recall normal stuff involving oil of some sort and a poke-y thing (brush on a thin/long metal stick, used to clear out any detritus inside the barrel), and in general you want to keep them in fairly cool/dry areas (i.e. in a gun locker, closet, etc. Metal can warp from persistent extreme temperatures, even treated stock can go funky if it stays damp. Not easily but when it happening can cause the barrel to explode in your face you don't risk it). Other than that, don't stick them barrel first into the ground, don't douse them in liquid, don't use them as a bludgeon, don't point them at people you're not trying to kill
ever. Probably covers the major points.