I bought and played a game for months and now xbox says I don't own it

"The thing is I bought it online, but not with a code, they lent me an account to install the game..."

Signing in to accounts that you do not own is against the TOS and can lead to permanent account suspensions as well as possible permanent console bans. Since you just paid to borrow an account that had access to the game, you do not actually own it. It will not show up for you. You were scammed. You should consider yourself lucky if you do not receive any enforcement actions taken against your account or console.

Without having any direct evidence, by your description, you obtained the game in a manner against the rules

Games and codes may only be purchased from authorized retailer, such as the Microsoft, an Xbox store, Amazon, Best Buy, target, Walmart, etc. If you’re purchasing a code and it’s not from Microsoft, or those kinds of retailers, it’s probably not authorized.

Your progress is likely still there, once you repurchase the game

Hi Aaron. My name is Kyle and I'm an independent advisor.

What device are you trying to play Minecraft on?

If you are using a Windows PC, please ensure you are signed into the Xbox app and Microsoft Store with the account which owns the game.

Also, please let me know of any troubleshooting steps you may have tried so far, and provide screenshots of this issue if possible.

Thanks,
Kyle

Oh, nevermind. It turns out that I’d already installed it with game pass but it expired, causing me to not be able to play Java. I’d already bought Bedrock, so I think the store thought I’d purchased the bundle. We’re all good now

Hi Tyler, my name is Rodrigo. I'm an Independent Advisor and a member of the Microsoft community and I'd love to assist you today.

Please, open the Microsoft Store on your computer.

Then, click on Library > Games.

On the far right corner, click on the drop-down menu and uncheck "Show only installed Apps".

I hope this helps.