What are best practices for cleaning and passivating stainless steel pot/fermenter?

Passivation is a surface layer of oxide that prevents rust. If you don't have rust you don't need to worry about passivation. I bought a cheap kettle that began to rust around the welds after the first use. I used Barkeeper's Friend to repassivate it and now it is fine. I use a sponge with a soft scrubber and clean it before anything dries on, so simple wiping is good enough. If you have a fermenter with a dried krausen ring, soak it with one of the cleaners mentioned above (dissolved in hot water) until the crud comes off with little effort so you don't have to worry about doing damage by hard scrubbing.

Last thing first. There is no need to worry about passivating the stainless steel. Once cleaned it will re-passivate when in contact with air and the residual moisture from cleaning. Its almost instantaneous.

Non-iron containing scrub pads are required for mechanical cleaning. Green scrubbies often found with sponges are fine. For less abrasive cleaning a sponge should work most of the time.

Cleaning agents are many. Check the labels for safety with stainless. That said stainless is pretty resistant to normal contact time cleaning. In the homebrewery, hot soaks in PBW or oxiclean work fine and usually require minimal mechanical cleaning after wards. For stubborn beer stone of the like Bar Keepers friend works great as a spot cleaner. I have also used CLR in a pinch as well. Just be sure to use it at the recommended dilution. Probrewers will use different regimens of acid and caustic depending on what the soil being removed is. I point this out only to emphasize the durability of stainless steel.

All that being said, really hard cleaning of a stainless fermentor is all I'd worry about. PBW and a good wipe out of the kettle is more than enough. Microbially speaking the kettle is getting sanitized during the boiling of the hot wort.