Of course, you can always manually backup the internal storage. But it is possible to include the internal storage data in a TWRP backup by temporarily renaming the media folder inside the root folder data.
A TWRP backup includes the TWRP folder for it is part of the internal storage. Therefore it will also backup all the large files which are present. It is therefore more convenient to keep the TWRP folder as small as possible, but it is up to you.
1. Go into TWRP recovery.
2. The path is button advanced button file management within the data folder you will find the media folder. Rename the media folder
temporarily to e.g. ABC.
3. Perform a new backup in TWRP and give it a new name. Don't leave TWRP but enter file management again.
4. You will find a new media folder. This is because TWRP creates this media folder, it includes your new backup.
5. Your follow-up actions will be to rename the new media folder into e.g. DEF and rename the temporary folder ABC (which is the original media folder)
back to media.
6. Restart your system and now you find a TWRP backup including internal storage in the DEF folder. Move this backup to the TWRP folder and delete
the DEF folder.
Do not forget when you perform a recovery with this backup to repeat step 6.