3.5mm jacks and what they fit?

Am I correct that 3.5 mm. jacks are designed so that they work with TR,
TRR, and TRRR plugs.

That is, the jacks intended for stereo sound and a mike will also work
fine with just a stereo speaker plug, and also with a monaural plug?
(Will it just have the left channel or will it combine the two?)

And the jacks intended for stereo sound will work fine with a monaural
plug? (Will it just have the left channel or will it combine the two?)