I've been trying to get a session that uses bspwm to be displayed as an option in sddm, but I can't for the life of me find where SDDM actually looks for its sessions that populate the menu! By default on KDE Neon 5.17, there are two sessions available: plasma, and plasma(wayland). I navigated to /usr/share/xsessions and found a single .desktop entry for plain kde. Does anyone know where sddm actually looks for sessions? Whats even weirder is that inside system settings, for sddm, it detects bspwm as an option, and I didn't even do anything to make that happen!

/usr/local/share/xsessions Not shown in the configurations files or the kde documentation but there is a file in /etc/sddm.conf that contains a single line that overrides a previously defined xsesssionsdir environment variable. That variable was /usr/share/xsessions which was why I was confused.