Finally figured out how to type 'special characters' in nixos+niri.

1. First set the compose key in niri config, e.g.
   options "grp:alt_shift_toggle,caps:ctrl_modifier,compose:rctrl"

2. To see all symbols:
   less $(nix-build '<nixpkgs>' -A xorg.libX11 --no-out-link)/share/X11/locale/en_US.UTF-8/Compose

3. Common symbols (for me)
   <Multi_key> <period> <equal> : "•" enfilledcircbullet # BULLET
<Multi_key> <s> <o> : "§" section # SECTION SIGN
<Multi_key> <asciicircum> <underscore> <n> : "ⁿ" U207F # SUPERSCRIPT LATIN SMALL LETTER N
<Multi_key> <v> <slash> : "√" U221a # v / SQUARE ROOT
<Multi_key> <colon> <minus> : "÷" division # DIVISION SIGN

To use these I type rctrl . = to get a • and rctrl s o to get §.