Initial betas of macOS Catalina can only open the previously selected iTunes library on your Mac prior to updating, according to Apple. Before you install the macOS Catalina beta, choose the iTunes library that you want to use by fully closing iTunes and reopening it while holding down the Option key.
Apple split iTunes into three apps on macOS Catalina: Music, TV, and Podcasts. The iTunes library that you choose will be the one that opens in the beta versions of Music, TV, and Podcasts on macOS Catalina. Your other music files, media, and additional iTunes libraries will remain wherever you have saved them.
Apple's support document suggests that multiple iTunes libraries will be supported in the general release of macOS Catalina this fall.
iTunes has long allowed users to have multiple libraries. Borrowing Apple's examples, you could have a library of holiday music that would not appear in iTunes the rest of the year, or you could keep your music in a library on your computer and your movies in a library on an external storage device.