What scenes are usually depicted in mosaics? are they still used today?

This depends on the time - mosaics were used in many different times in history.

For example, the Romans often portrayed their mythodology - check for example the attached roman mosaic from an Austrian museum - it portrayed the labyrinth and the story of Minotaur.

So a very common there was mythodologyOften, fruit or animals were portrayed. Sometimes - abstract symbols.

And yes, mosaic is still used today - I've seen it in an  underground station in Vienna recently!