As you have already been told, real singers don't use amplification. That's how you can tell the difference between a sunger and a poseur who can hold a microphone. A professional opera singer, or even a competent amateur, can fill a theatre of not just 1500 but 2500 seats with sound, without doing any damage to his/her own voice, or to the hearer's ears. The same is true of actors in spoken dialogue. Nonetheless, there are lots of things which theatres, actors and playwrights did and do to make it easier for audiences - partly, in the case of opera, because of language issues, and partly because sung text is never quite a clear as spoken text.
These include:
1 The design of theatres to have good accoustics - i.e. for sound to travel well and clearly to all parts.
2 Non-verbal clues, which include movements of the hands and body, expressions, scenery, props etc.
3 Familiar plots. For example, may Shakespeare plots are recycled from earlier sources. and many Shakespeare plays have in turn been recycled into operas. It is easier to follow the detail if you understand the broad outlines.
4 Programme notes, so the audience can bone up in advance on what is going to happen.
As for people at the back not being able to see so well, that is perfetly true in a large theatre, and is one reason why prices may be lower there. On the other hand, the sound is often particularly good there because of the reflecting effect (and focus) of the back wall. Swings and roundabouts.