Can’t use my Digitakt at the moment but was just looking at this in the manual, detailed info is on pages 23-25 and 34 in the new updated manual.
Looks like to add Sounds from the global Sound Library into your current project’s Sound Pool, you go to the sound browser, select the sound, hit RIGHT arrow, choose “Copy to…” then “Sound Pool”
EDIT: Also, a “Sound” is essentially all the settings on an audio track. The sample, LFO settings, AMP & Filter settings, etc.