Thumbfinder

This tool does work with full links, but it also works with just the video's ID, which is a specific part of the URL.
If you wish to use that, or are running into issues with full links somehow, but you don't know where to find the ID, I'll walk you through it.

Open the video you wish to find the thumbnail for, and look at your browser's adress bar (if you're doing this from the mobile app, you may need to copy the link and paste it somewhere like your phone's notes app).

If the link looks like https://www.youtube.com/watch?v=ItMyxevtk2s, the ID is the part after the "?v=", aka ItMyxevtk2s.
If the link looks like https://youtu.be/XciMC1lg_Y0, the ID is the part after ".be/" aka XciMC1lg_Y0.

Some links will have another ? after the video ID, followed by some other string of letters and numbers. These are usually to indicate a time to start playing the video at, play it from a particular playlist, or simply help keep track of how many people click the link from where (this last one is often the case for youtu.be links). But, no matter what it's for, that's not part of the video ID. (But I do have this thing set up to remove that part on its own.)

Nothing's generated, no data is being exchanged, I'm literally just pasting bits of text together.

You, yourself, could manually find any Youtube video's thumbnail on your own, with just two things:

• The ID of the video in question
• The format of the URL that Youtube stores their thumbnails at, in the particular size you want

... And I hate having to look up what the URL looks like every time, so I want a tool that remembers it for me.
But all the ones other people make are either incredibly shady or die out, so I'm taking this into my own hands.

You can look at the code, with extra comments on how it works, here.