You might have more than one display manager installed in your machine. To switch from lightdm (the default DM in EndeavourOS) to, say, sddm because you want to swap i3wm with swaywm, you can use the following commands:
First of all, determine the name of your display manager running the following:
systemctl status display-manager
It will return something like this:
● lightdm.service - Light Display Manager
Loaded: loaded (/usr/lib/systemd/system/lightdm.service; enabled; preset: disabled)
Active: active (running) since [DAY] [DATE] [TIME] [TIME ZONE]; [HOURS]h [MINUTES]min ago
Docs: man:lightdm(1)
Main PID: 1087 (lightdm)
Tasks: 12 (limit: 13863)
Memory: 434.0M (peak: 937.0M)
CPU: 3min 59.006s
CGroup: /system.slice/lightdm.service
├─1087 /usr/bin/lightdm
└─1096 /usr/lib/Xorg :0 -seat seat0 -auth /run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
Then, using that name, disable the currently running DM (lightdm):
sudo systemctl disable lightdm.service
After that, enable and start sddm:
systemctl enable sddm.service
systemctl start sddm.service
After that command, your machine will immediately switch to sddm. However, it's best to just reboot after enabling sddm.