How does BTT work (in a nutshell)?

I am interested in using BTT, but I can't easily find a description of how it works. I want to be confident that it won't mess with my system in such a way that, were I to uninstall it, then my previous settings would safely be restored.

Also, I'm curious to know if/how BTT settings take precedence over settings established in e.g. System Preferences. Thanks!

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