Its literally open source you can see how it works yourself. Its. 2 of 2 multisig wallet that does submarine swaps to pay ln invoices:
Discussion
My experience with Muun is that it also has the lowest fees. Either 0 or 1 sat. In my ~30 transactions I have never seen higher fee.
Also it's the cheapest way to get sats from LN to onchain (you can set 1 sat/vB and then it's just price of one tx)