Windows help

Hey guys,
I recently built a new computer. Before this computer I had a PC from IBUYPOWER with windows 8.1 which I upgrade to windows 10.
I learned about OEM and that I might have to buy windows for my new PC - but before buying it I wanted to try and see if it would work on the new PC.
I plugged my SSD which had windows 10 on it to the new PC and everything booted up no problem - didnt ask me to verify didnt give me any errors.
My questions are:
1. does that mean that my OS wasnt OEM? and that I can install it wherever I want?
2. is it only working cause windows was already installed on the SSD - and as soon as I reinstall windows itll give me errors?
3. Depending on the answers to 1 and 2 - I would like to reformat the PC and install fresh drivers and windows. Can I use a windows 10 ISO or do I need to use a windows 8.1 and then uggrade to 10.
Thanks