why is my backup file 35GB and how do i clean that up?

So, it is unclear what you mean; are you speaking of your local IOS device backup on your Mac/PC or what iCloud has noted.

If the latter, the likely cause (and it would be the same for a local backup as well), a mixture of your on device photos, videos, installed apps, web browser caches, etc.

One thing you could do is go into Settings on your IOS device > iCloud and therein, uncheck those applications that you have backed up to iCloud. Also therein go into the Manage Storage and scrolling down through, if there are apps you don't use/need, go into them and use the appropriate Delete Data... or Delete App options. Admittedly, if you find you have ones you need, you can redownload them from the App Store.