hey everyone! because we're learning it soon, now i updated my page a little so it says why we say で for "being in a place", and why we say です. (or at least, this is the info i found out about it anyway.) i will write it in english below. i hope it helps a little somehow:

https://marbuljon.neocities.org/tutorials/jpgram/particles.html

in the past, で was "にて" which meant "being, existing", but it turned into で. so we also use it for "being/existing in/at a place". for example, "がっこうで" = "(being/existing) at/in school".

うちで テレビを みます
(being/existing at) home, television-watching
= watching TV at home

in chapter 4 we will learn a continuation of the same "existing, being" rule, except they don't explain it so well so here it my attempt from what I read:

when で comes in front of either ある (exists, has/owns) or ござる (is, exists, has/owns - we have seen this already in the phrase ありがとう ございます) the meaning of the verbs change and then mean simply "exists, is" without the "owning" part of it:

ある = exists, owns, has; will exist/own/have; to exist/own/have
で ある = exists
ござる = is, exists, owns
で ござる = is, exists

then the more polite/formal/longer/older forms of these verbs get shortened and simplified to the same ones that we are learning in class. the meanings of ある and ござる are so similar that sometimes they merge into the same thing (not sure if this is "sometimes" or "always"...). the leftmost is more formal etc, the rightmost is least:

では (で + は) = じゃ (is)

で あります = です = で ある = だ (exists, is)

で ありました; で ございました = でした = で あった = だった (existed, was)

the differences below are talked about in chapter 4 but now we actually know why they are how they are:

テレビがあります
tv exists; (i) have tv

テレビじゃない
television being not = it isn't a tv

テレビであります = テレビです = テレビだ
television being is = it is a tv