Which version should I use for an iMac Late 2009?

Hi everyone,

So I just got a $5 21.5" iMac Late 2009 and I was wondering which MacOS should I use for this one?

I'll be upgrading it tomorrow with 16GB RAM and 512GB SSD so it'll be smoother.

Also, do I just need to install the MacOS on a USB key and boot it into the iMac or is there something else I need to do?

Thanks!