Depends on in what context you use it. So, for example, "вчера я был на стадионе, где было МНОГО ЛЮДЕЙ", но " МНОГИЕ ЛЮДИ любят ходить на стадион."

Depending on the context. But mainly you can use both. E.g. Много людей пришло на митинг. - Многие люди решили выразить таким образом своё мнение о происходящем в стране))) Even in this case they are interchangible but don't forget about sequence. Многие люди решили - много пришло.

I have learnt - one is an adverb and the other is a pronoun - which one you use depends on context - how the context determines which one to use is clear only to good Russian speakers after long experience The best advice seems to be they are interchangeable and no real problem after all, so I shouldn't have worried

"Много" is an adverb. "Многие" is a pronoun.

Not to say English is completelly deprived of this: "I know a number of people"... "I know numerous people".