See also this fandom vim wiki page.

• :s/find/replace/, only search in current line, and replace the first matched pattern

• Default: only in current line
• In all lines: :%s/f/r/
• In selected line: :'<,'>s/f/r/
• In selected characters: :`<,`>s/f/r/

• Default: only first matched in given range
• match every one: :s/f/r/g

• Reuse the matched pattern:
  1. :s/f/&/

     & is the text that matches the search pattern.

     Example: :s/[a-z]/&1/g, add a 1 after each characte

  2. :s/\(a\)/\1/

     \(a\) makes a backreference to a.

     \1 is the first backreference, \2 is the second backreference, \0 is the text matched by the entire pattern.