how to choose what all to backup

Backup how - to iCloud - or using iTunes/Finder on PC or Mac computer.

You can automatically backup your iPad to iCloud. Once enabled, you can optionally configure/choose the elements that you wish to back-up from your iPad Settings.

To enable iCloud Backup:

Settings > [Your Name / AppleID] > iCloud > iCloud Backup > iCloud Backup - set to ON

To configure iCloud Backup:

Settings > [Your Name / AppleID] > iCloud > iCloud > Manage Storage > Backups > [This iPad] > [Choose Data to Back Up]

From the list of Applications (which appear in sequence of those that consume the most space), simply enable or disable those elements that you wish to include in your iCloud backup. Whilst you can enable/disable all data for individual Apps, you cannot be selective to the level of individual files.

In addition to the natively supported backup methods using iCloud or iTunes, there are third-party Apps that can be installed on PC and Mac computers with which you can manually connect and backup your iPad and iPhone. One of the more popular is iMazing - this being available for both PC and Mac:

https://imazing.com/

The standard backup is all or nothing, no pick and choose. You can back up some things such as the contacts list, photos, videos separately, but the standard backup either from the device to iCloud or on a computer with iTunes or succesors (latest MacOS) will be all or none.

Might want to explore the iMazing utility to see if it offers usable alternatives.