How do I free up internal storage if I don't have any apps downloaded? I have an Android Voyager, RCT6773W22B. It's version 5.0.

When I first got it, and years after that, I could download at least five apps downloaded at a time and have a great sum of pictures in my gallery. I would still have space after this. Over time, I would delete apps I didn't use anymore. If I wanted to download something else and there wasn't enough space, I would delete apps and/or pictures I don't need.

Now, I can't download anything due to internal storage. I have no apps, five pictures (not taken with the camera), and I've cleared the cache on everything except Chrome. It tells me my internal storage is completely full and I can't move anything to an SD card. What do I do?
There are a lot of reasons that can cause this:


leftover artifacts from uninstalled apps or updates
download content from apps (videos/ photos from chat applications, maps for navigation, audio files for language learning apps, etc)
currently kept open browser tabs (my chrome had at one point 50 tabs, it reserved ~1.5gb of space even if closed it but kept the tabs)


If you do a backup of your contacts and other important data, you can try to reset it to factory settings with a wipe of all sd/ internal memory. 
If you have a pc to connect your phone by cable AND the knowledge, you could browse the file folders and find the culprit (I would still recommend to clean the files over the phone afterwards but its easier to find what is taking the space)
You could buy as a well a bigger sd card as a temporary solution.