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A couple of questions about 의 used to indicate posession

I have been reading about 의 used as a particle to indicate possession and have got a little confused.I understand it when they said 수잔의 가방 Susan's bag or토니의 지갑 Tony's wallet.

But then this came up and I have got confused.

As seen above, the function of the particle 의 resembles that of the
English suffix -’s. However, there is one clear difference in their usages.
English allows the possessor + ’s construction, as in “Steven’s” or “Andy’s However, Korean does not allow the possessor noun ending with the
particle. Consequently, a sentence like 그 컴퓨터는 스티브의에요.“As for
that computer, (it) is Steven’s” is unacceptable. For this purpose, a bound
noun 것 “thing” typically appears after the particle, as in 그 컴퓨터는 스티브의 깃이에요.“As for that computer, (it) is Steven’s (thing).”

Please could someone try to explain this paragraph in less confusing english lol!

Is saying 스티브의 컴퓨터not like saying 그 컴퓨터는 스티브의 깃이에요.? Or does something like 스티브의 컴퓨터 have to have something like is old/black/broken follow it?

Another question is when using 내 or 제 can I say both 그 것은 나의 운동회에요 and 그 그 것은 내 운동회에요? and 토마스는 저의 친구입니다 and 토마스는 제 친구입니다.or is only one way right?

Additional Details:

Also when does 갓 ( thing) become 거 in the construction noun 것 (or 거). Is is it 갓 after vowels and 거 after consonants?

Thanks in advance!