Odoo Users technically log in and out each time they request information or click a button, so there isn't really a concept of "being logged in".

What we do is poll to check if Users have been active and use this information to present a status in Conversations, so that other Users know if people are "online", "offline" or "out of the office" (if the Time Off app is installed):

Check out the User Presence (bus.presence) model. 

See also https://www.odoo.com/documentation/15.0/applications/productivity/discuss/overview/get_started.html