As with most matters of law, it's an absolute rabbit hole. I follow a lot of YouTubers, and fair use is something that comes up an awful lot so I've kinda got familiar with it as a concept over the years.

The main two points to keep in mind is; does it affect the market for the original work? Is the amount of that work used appropriate and reasonable? So for example, if I'm doing a review on a TV show, I'm perfectly entitled to use a couple of screencaps of the programme in my review. It's more interesting with YouTube, where some users would just reupload the original work, with their face in the bottom corner "reacting" which is obviously abusing the system.

The other point about affecting the market for the original work, I see people just reposting instagram posts for example as again crossing that line of fair use. There's no transformative element to it, it's just clearly theft.

A good tool is the "Usage Rights" feature on Google Images, if you toggle that on it'll only show images that have been specifically labeled as fine for reuse (sometimes there might be limitations, such as using them for "commercial" work).