temporary delete apps

You perhaps refer to “offloading” one or more Apps? “Offloading” Apps can be a helpful method of creating temporary space for a software update:

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

This process temporarily removes just the App(s) - not your associated data.

If you have any concerns, ensure that you have a full iCloud or iTunes backup of your iPad before you proceed with the system software update (this is always a sensible precaution). Then, if anything untoward should occur, you can restore all you data.

How to back up your iPhone, iPad, and iPod touch - Apple Support

You should not lose any data unless you manually delete the App(s) from your iPad - when you will also delete associated data.

As already stated, if you have any concerns, ensure that you have a full iCloud or iTunes backup of your iPad before you proceed with the system software update. If anything goes wring, you have an insurance policy in being able to restore all your data.

How to back up your iPhone, iPad, and iPod touch - Apple Support

121GB or 121MB?

121GB of free space will be more than enough for any software update.

+7GB should still be adequate to successfully perform the system software update.