you're essentially giving pow shitcoins hash power in exchange for bitcoin. it's not very profitable since eth went pos. they allow for btc withdraws via on-chain or lightning.

the windows tool is here: https://github.com/nicehash/NiceHashQuickMiner

you're also trusting the various bundled miner applications too.

their lightning node is here:

https://amboss.space/node/037659a0ac8eb3b8d0a720114efc861d3a940382dcfa1403746b4f8f6b2e8810ba

dyor of course. i don't personally use the service any longer.