When a plugin is bypassed it doesn't process audio but it's UI is still active, knobs can be moved and everything else, it just doesn't process any audio. .
When a plugin is deactivated (like turning off it's power switch), it's completely off, doesn't allow changing any parameters, not turned on, doesn't do anything.
When a plugin is disabled, a placeholder reference is kept for it on the track to enable it later but it's completely removed from memory, releasing any RAM it may have been using when it was loaded in the other two cases above, or in normal activated use.