I've often thought it would helpful if the thing I was cooking on was close to as wide as the oven itself. In Australia ovens are usually 60x90cm. I often see and use American recipes because they're so common on the English speaking web and they quite often refer to sheet pans or baking sheets, which seem not to be a very common thing here. They look bigger than the types of things I can commonly buy here, which tend to be cookie pans that are really small. I used to think those American baking sheets were literally as big as the oven and slid in as racks but on further research it seems they're not actually that big and also need to sit on racks themselves and aren't as wide as the typical American home ovens.
I guess my theoretical baking rack would need it's rims to be less wide than the distance between rack grooves otherwise the food would touch the oven walls and baked goods that rise would might rise up to those grooves which would be no good either, but still that should only be a few cm. I actually sort of already have what I want as it came with the oven. It's a rack, that's not a wire and is a solid continuous sheet of metal that slides in to rack positions. The problem is, it always produced weird results when baking and seems to burn the bottoms of cookies and also has a large shallow ramp at the front that messes up what you can put on it. I read my oven instructions and discovered you're not actually supposed to cook on this thing and it's for catching drips. That's super weird to me since on occasion it's been used for this purpose accidentally and it's singularly unsuited to the task as any drips immediately bake right on to it and are impossible to remove and produce lots of smoke on the next use of the oven. I guess it's sort of better than nothing since I can theoretically clean that off when I take the rack out to clean it as opposed to the oven floor, but it's only marginally better since the effect of the baked on drippings is so thorough that it's near impossible to scrub off. Anyway, point is, while it's for whatever reason unsuited to the task presumably because of whatever it's made of and it's slightly odd shape, it's proof in my mind that the concept makes sense and can be done, and yet I can't find anything designed for this.
You can buy additional wire style racks, but seemingly not continuous metals sheets of appropriate size to fit in to the rack grooves.
It's an electric oven. I'm not actually sure exactly how the measuring is done when websites tell you standard Australian oven size so I measured the internal dimensions of my oven. It's hard to decide where I'm measuring from and hard to be very precise but it's about 60cm wide by 40-45ish deep and about 40-45ish cm high. I don't really know where the 90cm figure came from and now that I'm googling it again I think it was a google 'answer' that I'm not seeing anymore that was misleading. I think it's more that most of them are 60 OR 90 cm widw and they seem to not typically give height or depth measures (I guess they're always the same?). Almost every oven here is Electric, gas is rare except in super old houses. I don't think the gas vs electric idea relates to sizing, but I don't have a gas oven to compare. In any case, I don't have one.
Anyway, there's confusion on confusion here so I'm probably just adding to it. I initially thought that these 'sheets' I was seeing on internet cooking videos were what I'd call oven parts as you say, but then again, I've never had an oven come with any such part. It seems like they're more akin to oven accessories, just a pie tins and bread pans are. Nevertheless, even when I figured that out, I still assumed these sheets were for sliding in to the oven rack grooves, in fact I'm pretty sure I have seen exactly that in many of these youtube cookery videos. The trouble is that the better quality such videos that I'm more likely to watch, have some money behind them and even though they're sort of aimed for home cooks, they probably use professional gear a lot more than most home cooks. I went looking for these things in the standard consumer places here: large supermaket chains, special kitchen shops in shopping centres even big discount stores that sometimes have random kitchen gear and none of those places have rack sized baking sheets or anything called or resembling baking sheets in fact. I couldn't even find ones that sit on racks, that are actually the same thing as these American 'sheets' which was a big surprise because I didn't think this seemed like a particularly clever or sophisticated invention or the type of thing I'd expect to be culturally different, they're just heavy duty metal trays designed for ovens. These standardised size baking sheets seem distinct for their lack of being anything special, which is exactly what I want out of them. All I seem to be able to find are various baking trays marketed for specific kinds of food, or roasting tins. They're nowhere near the size of my oven width, even the biggest ones, they're usually high walled, they have special features designed for particular foods in mind like pouring spouts, or non-stick coatings, or divets for making muffins etc. etc. The closest has been 'cookie trays' or brownie pans, but they're much thinner than the things I'm looking for and much smaller. They seem to be designed for small batches of very small cookies. I think it's weird because I'm not aiming for something unusually ambitious for home cooking here. I'm making what I would think would be considered a pretty small batch for anything commercial, and just normal to large-ish for a domestic baking. Sticking with cookies because they're a good example. I typically like them to be bigger, and one of these cookie trays I can find could fit, maybe 3 big cookies. If I made ones closer to the size they seem to envisage, even then I'd be making like 6-9, maybe 12 if they were absolutely miniscule.
I'm ranting here but I'm just so surprised that this is somehow an obscure and specialist want. I feel like it isn't for the Americans, but then, I only have potentially misleading sources to go on having never been there. So I'm half asking where and if I can get them, and also half asking if they are as commonplace as I assumed they were. It is sounding like the full rack sized ones I wanted are a bit more unusual wherever you are, but at the very least I hope to figure out if the 'half' or 'quarter' sheets are something your average American has in their home, or if that's all just Babish and Serious Eats and various other popular media giving me the wrong idea.