Do some organisms outcompete eachother

Yes, Organisms compete for the resources they need to survive like food, air, water, space. In areas where these are sufficient, organisms live in comfortable co-existence, and in areas where resources are abundant, the ecosystem boasts high species richness (diversity).

yes. some organisms outcompete eachother.

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.