If I upgrade someone else's phone under my plan, can I remove the sim in my phone to use with the upgraded one?

Hello, there is another member under my plan that has a Pixel 3 XL that is paid off.

I also bought a S9 off Amazon. I want to know, is it possible for me to upgrade the Pixel 3 XL line via trade-in the S9 but keep the upgraded phone for myself?

Can I use the sim in my S10+ in the S21 even if the upgrade line was another members phone?

Please let me know if something sounds confusing.

Thank You