Sending bitcoin on chain has fees because it the chain is a limited resource and people bid to be included in the next block.

Sending sats on stacker.news from the balance on the site updates a number in a database which is basically free but also centralized and not censorship resistant.

The lightning network which is the opening of channels with an on chain transaction that then allows for off chain transactions that are much much cheaper but not free. Lightning network is still bitcoin, not a number in a database. Still censorship resistant and you can always move your money back on chain if needed without asking permission.

Also it’s important to note that bitcoin and sats are different denominations for the same thing. One is not cheaper to send than the other because they are the same. 100,000,000 sats is 1 btc. Like 100 cents is a dollar.