If using either the Flatpak or Snap packages, the sandboxing interferes with launching binaries outside of their containers. You can't see your normally installed editors because of mount namespaces (aka fancy chroots).
Both make use of xdg-desktop-portals by default, which means if you press control-e for example to open an external editor, you'll get a list of editors to use assuming that functionality is working (reliant upon having xdg-desktop-portals installed on the host system, which is often default these days, and the editors having .desktop files that actually claims it supports .md/jpg/pdf/etc
As a special case with the Flatpak, you can use flatpak-launch to potentially launch arbitrary programs on the normal filesystem, but I'm not honestly entirely sure how that works, the Flatpak repo has this example
Path:
/bin/flatpak-spawn
Arguments:
--host /bin/flatpak run --filesystem=xdg-config/joplin-desktop org.gnome.gedit
That's presumably launching gedit also installed as another flatpak, I don't know if this could point to the native /usr/bin/typora
Ultimately, the official AppImage doesn't have these problems and given this is the official forums, you're encouraged to prefer the AppImage if you're having problems as the other ones are third party support only.