I tried to set a mark using a digit: m1 and apparently that does not work, because later I typed `1 and I think it opened the last opened file or something else, I'm not sure. What does m1 do?Are marks restricted to letters?
What does `1 do?
mark
You can jump to numbered marks, but you can't set them directly. According to :h mark-motions:Numbered marks '0 to '9 are quite different.  They can not be set directly.   They are only present when using a viminfo file viminfo-file.  Basically '0 is the location of the cursor when you last exited Vim, '1 the last but one time, etc.  Use the r flag in 'viminfo' to specify files for which no Numbered mark should be stored.  See viminfo-file-marks.