16gb isn't an awful lot of storage these days, especially if your device comes with a lot of pre-installed and non-removable apps. The apps you use are getting progressively larger as functionality is added, which they can do because most devices now come with at least 32gb, which is still enough for any but the heaviest user.
The only thing you can really do is go through what's installed and see what you can do without. Most of the Google apps can't be uninstalled, but Docs and Sheets and (I think) Keep can. It's also worth looking at the data stored by apps you only use occasionally. You can clear that and free up quite a lot of space, although you will lose settings and logins - I wouldn't do it with games, for instance. You'll need to repeat the process periodically if you re-use the app.
You can free 3.1gb immediately by moving your audio files to the SD card - the system should still find them. If you take photos, make sure the camera app is set to save to the card and them move existing photos there too.
The other thing is to regularly go into Settings and Storage and clear the cache - that can free up 100 or so mb at a touch. I'm afraid that, with a 16gb and even only relatively few apps, regular housekeeping is going to be essential.
There are still a few budget devices around with this amonut of storage, but they're best avoided. Regard 32gb as the absolute minimum when you come to upgrade.
