Does leaving Spain nullify the NIE/European Certificate and other documents?

Hello,

I lived and worked in Madrid in 2022. I then left Spain and have been outside the country since. I understand that the NIE refers to the number and not residency.

However, I am wondering, if I were to return to Spain (different city) would I need to begin the administrative process again or would it be easier to get a job since I already have a NIE and social security number?

In theory, would registering as self-employed with my current NIE be possible?

Thank you