Password-Protect Apps MacOS Big Sur
From a quick internet search, I found this article that lists two apps:

AppLocker is free to block one app, with a $10 purchase to unlock full functionality. Take this with a grain of salt, though - the average review is one star
AppCrypt is a one-time $30 payment (25% off with their coupon code). It looks like you can download it for free from their website, but likely has limited functionality