Instead of "rotten" I should have probably used "fermented". They're dark brown, not dry but not in a liquid, individual anchovies that are starting to fall apart. They smell like a stronger version of fish sauce.

This but also with pepper corns?

https://www.youtube.com/watch?v=23QDS0xclw4

Yeah, rough-ground peppercorn.

Why not just use colatura di alici?

I prefer not to cook that because I like the taste of it very much. I use it like Roman soldiers used to use it - a liquid that you put on whatever you're eating, like we use ketchup now.

I think the Italian fish sauce is much softer and more refined than Asian fish sauces.

If it's the salted anchovies I'm thinking of what I'm thinking of (like in the yt video in your sibling comment), salted anchovies are a lot cheaper and more versatile (the downside is that more processing is required on your end). Colatura is great in sauces, but you can use whole anchovy filets to make a gilda in addition to adding flavor to a sauce.