There are 3 available swap features in Terra station swap:
Using the Terra protocol’s market swap, which burns Luna to mint Terra and vice versa. You can also swap stablecoins this way. This is the “market” function. Read more here: About the Terra Protocol — Terra Docs documentation
Terraswap: station swaps via terraswap pools.
Astroport: station swaps using astroport.
You can choose which one you want to use, though station usually shows you the one with the best return for you.
The market module can only handle so much swapping per day. Swap pools allow for more liquidity.
What is going on with Swap? Are you no longer defending the peg? It seems so as Swap is not available and Terraswap and Astroport does not offer $1 of LUNA for 1UST (more like $0.1 of LUNA)