VIM: c#_< with ":browse oldfiles"?

Answer 1

1. In the vim help, the notation c_ is a prefix to indicate a help topic that relates in some way to Ex command lines. So, c_#< is the documentation topic for the #< special filename.

2. See above; c_ isn't something you would type into a Vim command line. It's also not related to CTRL.

3. To use the :oldfiles command, type :oldfiles which shows a numbered list of previously edited files. To edit a file by number, say number 5, use the command :e#<5

The :browse oldfiles command conveniently merges the two command in the previous description to one command. With that command, all you need to do is type the number of the file you would like to edit, without having to use the :e#<n command.

I'm unsure why you mentioned "marks-viminfo-oldfiles" in your third question. The :oldfiles command is unrelated to viminfo and marks.