Hi,
the 2 default desktops on Pocket Reform are both Wayland (wlroots) based and don’t use the X server, Xsession etc. They do have support for running X apps, but that’s another topic.
The TUI login screen you see after booting or when logged out is tuigreet, which is in turn a module for the greetd greeter daemon. The greeter takes care of launching the desktop of your choice.
The MNT Reform Setup Wizard writes the default desktop choice to the file /etc/greetd/config.toml. Replace the /usr/bin/wayfire in line 9 with /usr/bin/sway to change the default to sway, and vice versa.