When looking ay space used by system, I found that AssetsV2 (in System/Library) account for 270 GB.
In this folder, 3 account for 260 GB and are apparently related to Xcode simulators:
Content is a bit cryptic. Does it relate to some specific apps I compiled and ran on simulator ?
Or is it related to older versions of Xcode I keep on the Mac ?
How to know what each forlder relates to ? Is it safe to remove ?
My iOS folder is only 9.13GB, and my watchOS folder is about 4.8GB, so Ive no idea how you've got that much data in there!
In my Xcode > Settings Components I have iOS 18.5 which is 8.84GB (which is close to 9.13GB), and my watchOS 11.5 is 4.68GB (close to the 4.8GB on disk).
My best suggestion is see what simulator runtimes you have in Xcode, delete the old unused ones, and see how much space that saves.
It may be that you've somehow got old ones on disk that Xcode doesn't know about (i.e. Xcode only lists iOS 18.5 but your AssetsV2 folder has folders within for 17.5, 16, 15 etc.), so it would be safe to delete the old ones from disk.