Snaps do not in any way autmatically swithc to “edge” unless you explicitly told snapd to do this for you with either
snap switch --edge <snapname> or
snap refresh --edge <snapname>
What are the snaps you see being switched to the edge channel without you asking for this ?
snap list |grep edge
By default you will always install and run only stable snaps unless you actively tell the system to not do this… typically these snaps have gone through a process of QA and testing in the edge, beta and candidate channels before they enter stable.
just like a debian package in debian might enter via experimental, go to unstable, then testing and later land in stable after all QA has been processed, there isnt so much of a difference, except that the design of snaps makes them completely independent from the host OS.