Where do VST3 plugins live on a Mac?
Either /Library/Audio/Plug-Ins/VST3 for all users, or ~/Library/Audio/Plug-Ins/VST3 for your account only. Both are scanned by VST3 hosts. The user folder is hidden by default, so open it from Finder's Go menu using Go to Folder.
Installing to the system folder needs an administrator password and makes the plugin available to every account on the machine, which is what you want on a studio computer with separate logins.
The user folder avoids the password prompt and is fine on a personal machine. Problems only appear when a plugin is installed in one and looked for in the other.
macOS can allow the file onto disk and still refuse to run it the first time. The symptom is a plugin that appears in the DAW's list and then fails to open, which looks like a corrupt install but is not.
Open System Settings, go to Privacy & Security, scroll to the security notice about the blocked item and allow it, then rescan in your DAW.
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Yes, on M-series Macs as well as Intel. If your DAW is running under Rosetta, keep plugin and host architecture consistent.
It is hidden. In Finder use Go, then Go to Folder, and paste ~/Library/Audio/Plug-Ins/VST3.