Nicehash (the app) is a convenience tool to manage miner executables -- it's mostly those that are flagged by AVs. You can connect most miner executables to Nicehash (the service) or other pools directly, and some are open-source.

For instance I GPU mine with https://github.com/ethereum-mining/ethminer and CPU mine with https://github.com/xmrig/xmrig, both built from source and I don't use Nicehash (the app). But I haven't looked at the source, so at some point I'm still trusting someone.