You take photos and videos on your iPhone, and you may find that the app you see is always at its last minute with the installation from one end. The idea is to delete the app when space is needed, but that will change when it comes to software updates.
Software update is executed in the flow of downloading an iOS package containing a lot of files / data to the iPhone and decompressing (processing to restore the compressed / archived package). In order to do so, you need to secure free space that is several times the size of the iOS package, and if it is not enough, you will not be able to start updating. If you use it at the last minute, you will not benefit from useful new features and bug fixes included in software updates.
Free space will be restored by deleting photos / videos and apps, but it doesn't have to be. If it is determined that there is not enough free space at the start of the software update, the app that can be re-downloaded from the App Store will be deleted (the data created by the app will remain the same).
If the shortage still persists, you may need to update using a computer function. Connect your iPhone to your computer with a Lightning cable, and click the "Update" button on the iPhone management screen of iTunes or Finder (for macOS Catalina).
To update the software on the iPhone alone, you need to secure several times the free space of the iOS package. However, when connecting to a PC, you need only a little more space than the iOS package. Use this method if you do not want to delete photos / videos or apps.