How to Free Up Space on Mobile Device
If using Google Photos, there is a specific way to free up space. Note that the risk is removing them from the cloud (losing all your photos) if you do not do this correctly. See [1]
Option 1
Go to Google photos app, sign in, and tap on your face icon upper right. One option that pops up is Free Up Space. The app is transparent in that it informs you that photos are backep up and they will not be trashed in the cloud - only your device. This asked me to erase 4000 photos at a time, so this is a bulk action.
Option 2
For more granular control, select photos, tap your face, and select delete from device. Do not salect plain delete. Make sure you understand the distinction.