Adobe just finished their 3 day virtual get together, and have most of the lesson content online. You do need to sign into a free account to download any lesson materials. My reactions were very mixed -- they had presenters who never taught; people who are happy & cheerful in every other video I've seen seemed upset or maybe depressed; it seemed like no one did a trial run to see how these 1/2 hour lessons would work in real time. That said, if you're interested in photo/image editing at all, you'll probably find something to like, or at least something that's useful.
adobe[.]com/max/2020/sessions.html
Now, Frequency Separation...
One of the classes, a set of three, 1/2 hour videos, covered what the author called FS2 [Frequency Separation 2]. The content is pretty good, though the presenter is inexperienced to put it politely, talking pretty much in a monotone throughout -- I didn't really get the gist of what he was trying to teach until the very end. FS2 is something that can really be Very useful. Unfortunately I've never seen frequency separation itself taught or explained well...
TO start with ignore the name -- forget about frequencies entirely. 2nd, think Completely 2D -- while your photos may show depth & dimension, it's a 2D picture you're working with. And you sort of take that picture apart. What you wind up with is a slightly blurred copy of your photo, showing all the colors but Very little detail or texture, and you get another copy that's mostly gray, showing just the detailed texture. Then you work with the color on one copy, the texture on the other, and finally put them back together again. The advantage is that without frequency separation you can't do a lot with say the clone tool or brush to fix colors without altering the texture, and you likewise can't fix the texture without messing up the colors.
You can do frequency separation in pretty much any image editor that does layers, and *possibly* in some that do not. While the video's I link to below are for Photoshop, the principles are the same regardless the software. The 1st couple of links are to basic FS tutorials at Photoshop Cafe, which I think are good for the basics, then the next 3 are for the class at Adobe Max that I think will show you how to actually put FS to use.
photoshopcafe[.]com/tutorial/frequency-separation.htm
photoshopcafe[.]com/frequency-separation-silky-smooth-skin-photoshop/
adobe[.]com/max/2020/sessions/frequency-separation-20-part-1-photoshop-fs20-reto-l6153a.html
adobe[.]com/max/2020/sessions/frequency-separation-20-part-2-taking-images-to-th-l6153b.html
adobe[.]com/max/2020/sessions/frequency-separation-20-part-3-problem-solving-tec-l6153c.html