Have you already tried slowing down videos using TikTok's built-in player or downloading them from special services?
The main lifehack is to use the slowdown function right in the app. You hold on the video, and there's an option for 0.5x or even slower. When you watch at half speed, you can see the hand movements, the direction of the thread, which loops the hook or needle goes through. I was once looking for a pattern with twists that I'd seen in my feed, and that's exactly how I figured it out - turned out it was just a twist made from two knit stitches, but in the fast video it looked like something complicated and magical.
If slowing down doesn't help, search by keywords in TikTok or YouTube: "crochet cable stitch," "knit pattern tutorial" plus a description of what the pattern looks like (waves, braids, bobbles). Often people have knitted the exact same thing and posted a full breakdown. Plus Pinterest has a huge database of knit patterns with detailed diagrams and charts - sometimes there's literally everything ever knitted there.
And yeah, sometimes just writing in the comments works: "Hey! What stitch is this?" - some creators actually do reply, even if they don't always remember the name. But counting on that is pretty risky at best.