If there are no provisions at all for multi-server, sessions can be stored in local files, which would lead to probably pretty poor performance if they're thrown on an NFS share.
I've been running multi-server Moodle for many years. My sessions are in Redis, but you have the option to have them on the database.
High traffic Moodle (mine is about 40k users) is a bit involved. You need to set up caching and also connection pooling in the database and there are a few hidden pitfalls, but it can be made to perform very decently