Under the Songs tab you can find tons of songs, including both Yousician songs (used in missions and past weekly challenges) as well as user uploaded songs. Here's an example from guitar;
Recommended - The most popular songs on Yousician; check out the Top 100 and Trending songs. You can also add your publicly available direct links to sound files, as well as cloud services and let SongCreator create the exercise for you.
Popular styles - Find some of the most popular styles of songs.
My Repertoire - Here you can find your uploaded songs, favorites, collections, as well as your last played songs under History. You can also challenge yourself and go collect missing stars and go for gold stars.
Confused about how to search for a specific song? Check this!