I created sheets for my players and now want to use them.

You can upload those sheets into a campaign slot, as they were yours. Then, you go into the campaign slot and click on "Unassign" on the characters. The players joining the campaign can then claim those characters.

What I don't see is a way to upload them in the interface. I see export but not import.

Go in here: https://www.dndbeyond.com/my-content/campaigns

And create a campaign slot. Upload all the sheets that you want with the link provided on the top right. Then, un-assign them.

Invite your players with the very same link and each of them can then claim a character you made.

Danke

This was still a little unclear to me until a moment ago. If you have characters in one campaign and you want to move them to a new campaign:

• First go to your old campaign and remove them from it.
• THEN go to the new campaign and copy the link you would "Send players this invite link to have them join your campaign".
• Paste that link in the URL of your browser and go to it; this will redirect you to the "Join a Campaign" page
• Click the character you wan to join with, then "Join with this character" to move them.
• This only works for one character sheet at a time, so repeat those steps for any other characters you might be adding to this campaign.

Also, this only works with characters you own. I just accidently deleted one of my players lol. 

So does the above process work if a player of mine wants to transfer one of their characters into my campaign?

Yes. They can have their character leave any current campaign. You can give them the Join link for your campaign and they can use that and will have the option to join your campaign with their character.

Thanks

Any idea what happens if Campaign A's DM has bought sources that Campaign B's DM doesn't have... And Player has used those sources to build their character.

The answer is it depends.

If the player uses the join link for Campaign B to create a new character for it, then they will only have access to sources that they own, or if content sharing is enabled, other people in the Campaign B own.

If they instead create their character inside Campaign A (with content sharing enabled) then they can use sources that people in that campaign own. They could then leave that campaign and use the join link for Campaign B, selecting the character that they made in Campaign A and then left. Note however that while this currently works, it may not in the future, as when you leave Campaign A you will get a popup that says it may break your sheet.

If you use content shared with you in Campaign A to build a character in that campaign, and then remove that character from that campaign and move it to a different campaign, that character will retain any content it already has on it. For example, subclass, race, or feats will stay on that character. If any of that content is removed from the character, it will lose access to that content.