Muun released that info on Twitter a couple years ago. You learn the nuances by asking questions, following good people on here, going to meetups, hop in a bitcoin spaces, podcasts, plebnet.org, and by making lots of mistakes.
Discussion
Muun tries to simplify by having one address to send and receive to lightning or basechain wallets. When fees are high muun gets exposed but not really using lightning. It's a workaround for ease of use.
Phoenix and Breez are running lightning nodes on your phone. They are cool but there is a learning curve. We are still in the very early days of this and seeing the speed of lightning payments is super exciting.
I completely misunderstood that app until today. I tried to move 10,000 sats to try a Phoenix wallet and the fee was so big (131,000 sats) that it wouldn’t do it bc i lacked funds! Possibly similarly bad I deposited $25 usd into strike to get some quick sats to play with Phoenix and Strike only gave me 9000 instead of 90000 sats. I still haven’t heard back from them regarding why that happened.
Yikes, that’s a lot of sats! If you don’t mind waiting (potentially a long time) usually you can set a lower fee manually. 
BTW. I think I have to correct my misinterpretation of the sats strike sent me. They send me 0.0009032 and I interpreted it as they shorted me. I thought this bc the last decimal was missing and bc I had visually compared it to my last 25 dollar order of 0.00090914. Not that you care but I didn’t want to let it slip that I made a mistake.