What is the best way to store sessions in a database or files?

Question

Sessions

What is more reliable and scalable?

Answer 1

The db is the file(s). It's just that a finite set of operations has already been defined on these files.
Reliability and scalability parameters depend on the specific database.
Sessions are no different from other data. If the loss of sessions in the last N minutes is not a tragedy, then you can store in redis, saving data to disk every N minutes.