[ ] is easy. It's just the 'test' command.[1]

  % which \[
  /usr/bin/[

All of the other syntax is shell-specific (run via built-ins), so that can vary, and be a little more obtuse.

[1] Albiet with some minor differences:

  % if [ -d /tmp ]; then echo "TRUE"; fi
  TRUE
  
  % if test -d /tmp ]; then echo "TRUE"; fi
  test: too many arguments
  
  % if test -d /tmp; then echo "TRUE"; fi 
  TRUE

  % diff /usr/bin/{test,\[}
  Binary files /usr/bin/test and /usr/bin/[ differ

  % man \[
  No manual entry for [