VST3 SDK docs are online here: https://steinbergmedia.github.io/vst3_doc/vstinterfaces/index.html
In order to debug hosts that have been code-signed with the hardened-runtime entitlement, you need to re-sign them. talaviram has made a nice shell script here: https://gist.github.com/talaviram/1f21e141a137744c89e81b58f73e23c3