pods get into your tank from the live rock or coral you add to your tank and they just multiply, or you cna buy them and add it to your tank. As for the white shrimp things they can be copepods, amphiods or maybe even mysis shrimp
If you can see them swimming and crawling around, they are MOST LIKELY amphipods. Copepods are extremely tiny and can easily be overlooked. If you have LR, then you are likely to have copepods in there.