Using a propogator

I bought a cheap propogator with seed tray (has 40ish compartments) and it has a plastic clear lid also to grow some seeds so some questions -

If seeds say grow direct would they benefit from starting in a propogator first?

How many seeds do you put in a little compartment - I assume this is different for larger or smaller seeds?

Would it be better to put water in the bottom and let the soil soak it up or water from the top?

I have already planted some seeds and if I have 3 little plants growing in 1 compartment what do I do next. Where little seeds were used I may have 20-30 tiny plants in 1 compartment... so what do I do there?