Well, here's how it works, LOL! When placing new mulch around plants already in the ground, put it all around the plant but leave a few inches, at least one inch, bare around the stem. You can damage or cause the stem to get diseased or rot if it's right up against it. For areas that I seed, I usually place the mulch and scrape away small bare spots, or sow my seeds and wait for the plants to get large enough to put mulch around. Perrenials should come right up through the mulch each year. And self sowing plants usually come up just fine, maybe not as prolifically as on bare ground but that can be a good thing
Some plants do very well creating their own "mulch" just by self shading the ground below them.
I am sure others will chime in with lots of good ideas. Good Luck!!