Architectury API is an optional intermediary library which mods may utilize to ease developing multiplatform mods.
Architectury API is an API to abstract calls to Fabric API and Forge API as both loaders have different implementations and interfaces.
Architectury updates regularly, with new hooks and features. Currently contains over 80 events hooks, networking abstraction, loader calls abstraction, game registry abstraction and more.
OptiFine support is not guaranteed with Architectury API. You may attempt using the latest version of OptiFabric / OptiFine with the latest version of Architectury API.
There are a lot of excellent OptiFine alternatives out there in the wild these days, you can mostly follow this guide.