No, ll does not mutate to l - it is the other way round.
There is a table of soft mutations in the course notes. There are also many on the web such as:
To find the course notes generally, go to https://forum.duolingo.com/topic/924/hot and see 'Course tips and notes'. The 'duome' link there is useful for browsing all the notes in one place. We recommend reading the notes for each new section as you start it.