I use iPhone 6s 64 GB variant. Here's how I manager my storage.

1. For photos I usually back them up to my computer using iTunes or by directly connecting it to my computer to backup the DCIM folder. Then I remove any photos I do not want to keep on the iPhone to make some room to take more pictures or download media from apps. I try to stay away from iCloud for backing up photos. Self storage of your photos is good in a way. If you feel otherwise, try using iCloud as a solution.
2. Offload unused apps - Your app data is preserved on your phone but the apps are in a way removed to recover storage space.
3. Go to settings > general > iPhone storage and see which apps are using up more storage space. Deleting an app removes the app + data. Offloading will remove just the unused app but retain the data.
4. Clear safari's browser cache/cookies or browsing data regularly since you have just a 16GB capacity.
5. Delete old mail storage and iMessage / messages. You can setup messages to keep data for a particular period of time and auto delete.
6. Also see if this helps to clear up some 'system data' - https://www.reddit.com/r/ios/comments/u8jo49/comment/i5ll7sa/?context=3 . iOS usually handles this automatically but this is a workaround as a manual approach.