How to get These Books on your Preferred Reading Device

Since the majority of readers use Kindle, I'll start there and go down the list to other eReaders that are common.

Instructions for Kindle: Luckily, this is quite straightforward. Navigate to https://www.amazon.com/sendtokindle, login if you're not already, and drag/drop the file into the box or click the box and navigate to where you downloaded the file and select it. It may take a few minutes for the book to appear on your device, so just be patient and close/reopen Kindle if needed.
Instructions for Google Books: This isn't quite as straightforward, but not too complex. Seems this needs to be done via a desktop computer, or perhaps switching to the desktop site on your mobile device. Navigate to https://play.google.com/ and click on your profile picture, then select "Library and Devices". Click on "Books" at the top, and then look for the "Upload Files" option (top-middleish). From there, you can drag/drop the files into the popup box or click "Browse" and find where you downloaded the file. The book should appear on your device fairly quickly under your library in the Google Books app.
Instructions for Kobo: For Kobo, there are two options. The easiest of the two is by using https://send.djazz.se/. I haven't tested this, but seems simple enough. Navigate to that page with your eReader device and then follow the instructions.

However, if you'd prefer to do a more official route, here are the instructions from Kobo to upload via your PC:
https://help.kobo.com/hc/en-us/articles/360024775093-Add-non...
Instructions for Boox: This is also quite straightforward. Navigate to https://push.boox.com/, make sure you're logged in, then just upload the ePub file based on the instructions. I don't have an account, so I can't see exactly what it looks like after logging in, but I presume it's similar to Kindle/Google.
Instructions for Nook: I found some instructions from B&N on their official site. It seems fairly straightforward, but if you have any issues, let me know: https://help.barnesandnoble.com/hc/en-us/articles/5464284834...

Here's an additional link if you have a GlowLight device:
https://help.barnesandnoble.com/hc/en-us/articles/5599565083...