possibly several parents, like 8 parents

Fun fact. Such merges with more than 2 parents are called 'octopus merges'. The Linux repo has a single merge with 66 parents that Torvalds named the 'Cthulhu merge'.

Git never deletes a commit that is less than 90 days old.

On its own, that is. Not if you do a git gc.

Deleting a branch removes a reference to a commit, but the commits in the branch are still there.

but you can see them using “reflog” related commands

Reflog - one of the most underrated git commands that has the potential to save your life some day. At least one team member must learn it.