Hi and welcome,
You are right, Cubase doesn’t support AU plug-ins. But most (I would say all) manufactures support VST.
Cubase supports VST3, what is the latest version of this standard and it’s recommended. But Cubase also supports an older version VST2 (it must be VST2.4 and newer). Several manufacturers still produce this old VST2.4 plug-ins (unfortunately; in VST3, they have much more options). No worries, these are compatible with Cubase.
VST2 plug-ins are using *.vst extension, VST3 plug-ins are using *.vst3 extension.
If you want to check, which VST plug-ins are installed at your system, have a look to these folders:
system/Library/Audio/Plug-Ins/VST
system/Library/Audio/Plug-Ins/VST3