Android doesn't let you copy and paste files into the data file anymore, so you either have to use a computer or the original files app(Even apps with "Manage storage" permissions can't copy or move files to it normally)
(There are a few other ways, but those two seem the easiest, and you don't have to download anything else)
To get to the original files app go to Settings ->Storage-> Files
then open the Files app when it asks you which app to use(Not Files by Google)
Then follow the instructions on the linked post(You basically have to drag and drop instead of "copy/move to" commands)
You want the "Copying and Moving Files into the Folder" heading
It worked for me, but who knows with other phones