he switches between them and I don't understand why
Here's what Subdivide Edges and Collapse Edges do:
The same stroke made with different settings.
Subdivide Collapse does both.
Subdivide Collapse is usually your best option, but when ('famous' use case) Pinching a Crease for instance, you will want to switch to Subdivide Edges.
I recommend playing with those settings for some time, while having the Wireframe (set to 1) Overlay enabled.
Also, there's a couple of courses where Kent explains the differences and uses in detail of each method. If you didn't understand the first time you watched, that's ok, you can watch the lessons a couple of times as you practice. That's how I had to do it because Blender is a huge beast and it's hard to fit it all in your head the first time. In this lesson Kent goes deep into the explanations:
https://cgcookie.mavenseed.com/lessons/dynamic-topology-2-8
Again, don't be afraid to go back and watch it a couple of times and most of all, practice along as he is explaining, at least that's how I do it and the knowledge solidifies in my brain much quicker.