They are not deleted, it has nothing to do with it.

Of course, if you upgrade to beta software, it is advisable to back up your data.

Sometimes it does, when the update detects faults on your device. In this case, I recommend updating it with the external tool Tenorshare ReiBoot, which not only repairs your phone without losing data, but also helps you in your case: updating it.

Depends on the way you update, there is the Over-the-air option which downloads the update to your phone directly, this way everything stays the same, the only downside is that you need to have enough space on your phone to download and update the files, The iPhone itself tells you if this is possible or not

The second option is to download the update through a computer, in this variant technically if the data is deleted however before updating you can create a backup of these and when you finish the update decide if you want to load that data again or start from scratch