Internal shared storage
This will give you a breakdown of what is on the device, separate from any SD card.
First on the list should be
Photos and videos
If there is much here, it should all be moved to a SD card or other storage.
(I do this at least once a week.)
Come back to that later.
Scroll down and click on
Then click the overflow (three dot icon) in the upper right.
Now all apps should be listed by size, largest first.
(If not, click the overflow again and click on Sort by size.)
This is a good way to see what apps are being space hogs, usually with caches- but some will also accumulate lots of data as well.
Some are safe to clear all data, such as Google Play Services, because they will get whatever they need to work on their own.
Others, like most apps that you have installed, will lose all personal settings if more than the cache is cleared.
(Caches are always safe to delete.)
Keep in mind that most apps grow over time, including system apps.
This happens due to updates and useage.
So, eventually your apps will grow to the point that the device starts giving you grief over it.
At this point, you have some decisions to make.
Do I really need this app that I like a lot but rarely use?
How about those 10+ small but hardly used apps?
Should I just purchase a different device with more memory capacity so that I can have all of these apps?
I have often been in your situation, as I always have cheap devices.
(Not that you do, but the end result is the same as far as space constraints go.)
The best idea I have found is to keep the internal memory about 70% full at most, because this gives an ideal amount of 'breathing room".
Apps grow and shrink as you use them, and if you make more space you will find that certain apps might grow even larger than you have ever seen them be before.
This means that you have been hampering the app's performance by having a device that is too full.
(Not that I am in any way a fan of apps growing large or accumulating data, but it is what it is.)
It has taken me multiple devices to get to the point where I have what I need and still have room to make a couple 2GB downloads in a row and not immediately need to make room for them.
So, if any of this helps, great.
If not, then go back to the internal storage, and take screenshots of what you see and post them here.