Fluval U2 Internal Filter Carbon Cartridge

It's my first time, Do I replace both at the same time or will it cause my tank to go through a mini cycle? Are useful bacteria colonising in these?

Do I have to remove these cartridges from the filter before using medication treatment?

Does this help reduce nitrates?

As you replace the filter, it’s sponge then the filter. It the carbon of the white side uppermost. Very confused.