What are these circle marks on injection molded plastics?

No, they are not gates, they are from pins that extend and eject the moulded part from the mould when it opens. That's why they are round and flat. The sheer number on that moulding indicates it is hard to release.
A cold well is a cavity into which the first injected plastic (it has cooled out at the injection nozzle between shots) can be held back, allowing properly heated plastic to enter the mould. Temperatures are important.

These are called gates where the molten plastic was forced into the mold.

I think that is the point where the melted plastic was injected into he mold. After the plastic cools they have a process which removes the post of plastic which is attached to the piece where the marks are. As for the cold slug, I have no idea.

those are the sites where the plastic is injected into the mold.