The car is a white Toyota Camry 2012.
As you can see there are many tiny black dots on the roof. I can't remove them by scratching by hand or with a high water pressure cleaner.
What caused these dots? What can I do to remove them?
Looks like tar spots to me - unusual to have that many on the roof but not impossible.
Try a dedicated tar remover - Autoglym do a good one (called "Intensive Tar Remover" IIRC) that should have a good chance, it also works well for tree sap (which is another likely culprit).
Looks like tree sap indeed.
The best way to remove it is to use a clay bar after a good pre clean without putting too much pressure on the paint.
The cause can be one of many possibilities:
or worse paint overspray
The first two should come off - good cleaner / polish and lots of work, but the paint may mean that you need a re-spray at the worst - there may be a professional treatment that could do it. So go and see a good paintshop and see what they say.