To help prevent unwanted overwriting of system files, macOS Catalina separates your main disk into Macintosh HD and Macintosh HD - Data. From each volume's point of view, so to speak, the other is included under "other volumes" when viewed in Disk Utility. I've not seen it shown that way in About This Mac > Storage, but I can only guess you're describing the same thing. So, the "other" usage shown against Macintosh HD is your documents, pictures etc. and the "other" usage against Macintosh HD - Data is the operating system and applications. No additional space is being used, it's just organised differently behind the scenes as of macOS Catalina. In Finder, it all appears together as Macintosh HD. You can free up space in a number of ways, including of course by removing data or apps you no longer need.