question on storage that is taken up when completing the process of boot camp

im a bit confused though, why would i need then a 16 gb or larger flashdrive to do it using a flashdrive? because apple says you can use just your computer with no external flash drive or (if not supported) then you'll have to use your computer and a flash drive or something and they specify the flash drive needs atleast 16 gb or more but from what your saying, you cant use an external drive for windows? is there any clarification you could maybe give me by any chance on this in general (involving the flash drive side of things) (if not thats ok) (oh and im also aware now that the majority of things will take place on the computer, but im just confused on why the flashdrive is needed for older modeled computers)

sorry about the punctuation, i did recap quite frequently and if anything its a lot to ask, but basically i was wondering exactly how much it takes up overall, like, if i were to use a flash drive for windows, i was asking for peoples input on how much storage it takes up exactly for them personally on the flash drive, and how much storage it takes up on the actual computer itself (because you cant just make it use the flashdrive entirely), and i was asking because while yes, i need to partition anyways, i was curious on how much it took up for both so i knew just how much i should partition (ignoring the recommendations by apple), because i would partition only 64 gigabytes or something, but is that really enough? is it really going to need that much? idk, and i was explaining how i tried contacting apple about this but they weren't helping much because they were just all sending the same article that i already read before even contacting them to begin with, and it didn't even answer my question because i was asking how much it took up after the process was complete, not the recommended amount of storage and stuff (again, sorry about the punctuation)

ok that actually helped a lot with pointing in the right direction, but im still stuck on one thing, that is that it says you can use an iso image of windows that is downloaded, but on the contrary, it wont let me on my old mac, so like, your answer lines up with how its acting (not detecting the iso), but it doesn't say on the website that it cant use the iso, do you by any chance have an answer for this behavior of it? (its an old 13 inch late 2011 macbook pro)

Oh wow ok, i'll try that, i'm going to guitar center for my guitar lesson's (i've been playing for like, 4, 5 years now, i don't remember which one) in 30 minutes and after i'll pick up a dvd-r SL or DL of windows 8, whichever one happens to be at best buy and see if it works (also, with my own research, i found that window's 10 does not run on a late 2011 13 inch macbook pro, so i'll just get windows 8 if they have it), thank you and that clarified so much for me