Hello,
It would be nice if videos added to a song can stay on a external ssd drive. Now you have to buy a expensive ipad with a large disk and external drives are a lot cheaper.
Sorry, this is a limitation of iOS -- apps can only read files from their own data directories, which can't be moved. As a side note, Android has adopted the same limitation in version 10 and later.
Would this feature be a possibility for macos?
Not that I know of. The macOS app works the same way, saving all downloaded files into a folder that the OS has designated for the app. This is all meant as a security feature.
For the Android version, since my last message I found that Android can use an alternate data directory on a removable SD card, and I added Settings > Account Sync > Use Removable Storage to enable that. But I haven't seen something like that for macOS.