Probably a stupid question but about phones?

I have an iPhone 6s, and I want to buy and iPhone 8 Plus, which I want to buy the cheapest option (so there be little 4g) and use my current SIM card in the 8 Plus and put the 8 Plus SIM card in the 6s and give it to my younger brother. I'm sure I can do this but I just wanted to make sure.

Yeah you can