CLAP plugins can be loaded in an effect slot using the slot's down arrow. The CLAP plugins appear in the effect selector's Plugins section.
Windows: CLAP plugins are files, the name ends with .clap.
64 bit plugins are located in the C:\Program Files\Common Files\CLAP\ folder.
32 bit plugins are located in C:\Program Files\Common Files\CLAP\ (if your Windows version is 32 bits), or C:\Program Files (x86)\Common Files\CLAP\ (64 bits Windows version).
64 bits MultitrackStudio can only use 64 bits plugins, 32 bits MultitrackStudio can only use 32 bits plugins.
Mac: CLAP plugins are bundles, the name ends with .clap. They're located in the user or system Library/Audio/Plug-ins/CLAP folder.
You can create the CLAP folder yourself if it's not present.
For plugin developers
The CLAP plugin API is available at https://github.com/free-audio/clap
Almost all extensions are supported, except for the draft ones. The draft plugin-invalidation extension is supported.