new windows installed, gap between masonry and windowsill

Good evening,
can you tell me how I can best close the gap between the window frame and the masonry? The gap is about 4cm wide and 10cm deep.

Do you also have a tip on how I can install a wooden window sill? There is currently no support surface. On the contrary, the wall is currently deeper than the window.

Thank you very much in advance!

Phew. Window installation...
Google RAL. Window hinges, foam, compression tapes, sealing levels, inside more airtight than outside, etc.

If you already ask a question like that, I fear some really bad things.

Regarding the windowsill. Was that once a heating niche? Will it be closed off again - open is bad since you can’t even properly place anything in front of it. So brick it up with Ytong or similar.
Then you can simply glue the windowsill on. Otherwise, you have to work with brackets.