Hello @marmeaux. I hope this helps.
For my step 1:
a. Open Finder
b. Look on the left side for "iCloud Drive" and click that. You should now see a list of folders and files to the right.
c. In the list of folders on the main (right-hand) section, look for Pictures and click that. You should now see a list of folders and/or files that are in the Pictures folder.
d. Look for a file called "Photos Library.photoslibrary". Depending on your settings, it might just show as "Photos Library".
e. If you see this file, you have successfully completed Step 1. This is where all your photos are stored locally.
Step 2:
a. Open another Finder window (from the Finder menu at the top of your screen, choose File and then New Finder Window)
b. In the left pane, look for "Pictures" under Favorites. If you don't have that, look for Macintosh HD, then choose "Users", then choose the folder with your account name, then look for "Pictures" under that.
c. The Pictures folder you found is where you want to move your photo library in the next few steps.
d. Position both Finder windows so that you can see the Photos Library file you found in step 1 and the Pictures folder of both on your screen at the same time. Resize them if necessary.
e. Move your cursor to the Photos Library and click on it once to select it. Then click and hold so you can drag it. Move your cursor a little to make sure it's getting dragged.
f. Drag the file over to the Pictures folder you found in the other window, and drop it into Pictures by releasing your mouse button (or raising your finger on the touchpad).
At this point, you'll get a warning about the file not being available on other devices. Don't be alarmed; this file is only local storage so you really don't want it in iCloud anyway.
Note: There are probably easier ways to do this, but this is the most explicit set of instructions I could think of that will work for everyone and doesn't require using terminal. If someone else has a better way, I'd love to hear it.