Learn Turkish: O dönemde halk zengin değil, fakirmiş. - In that era, the public was apparently not rich, but poor.

The suffix -miş is the reported or inferential past tense. It means the speaker didn't witness this era firsthand but heard about it, read about it, or is inferring it (translating to 'was apparently poor' or 'I heard they were poor'). If they lived through it and saw it themselves, they would use the direct past tense fakirdi.

Does the 'apparently was' meaning apply to zengin değil too, even though it lacks the -miş suffix?

Yes! In Turkish, when you list two or more adjectives or nouns like this, you only need to put the tense suffix on the very last word. So zengin değil, fakirmiş means they were 'apparently not rich, (but) apparently poor.' You could say zengin değilmiş, fakirmiş, but dropping the first -miş is much more natural.

Why is the ending singular (fakirmiş) when halk (the public/people) refers to many people?

Even though halk refers to a group of people, it is grammatically a singular noun in Turkish. Collective nouns like halk (public), aile (family), or takım (team) take singular verb endings unless you make the noun itself plural (like halklar, meaning 'peoples').

How does O dönemde translate to 'In that era'?