Birth control and plan B?

This is just a question because I don’t know. I’m not on the pill but will be soon and feel like I should know this information ahead of time.

I read somewhere to not take plan b if you are on birth control?? Is this true? Why?

When I’m on birth control (I’m also planning on continuing condom use), should I be concerned if the condom breaks and we didn’t notice or he finished in me or whatever?

If the scenario above happens, and I get my regular period (bleeding for 7 days) on time, I shouldn’t worry right?

I heard of certain stuff I should not take with birth control like acidic foods, antibiotics, melatonin. what else should I avoid?