Got my visa! Now what? What happens a year from now?

Hi everyone,
I recently received my visa, yes traveled all the way back to the states to get it and finally, I am legal! Well the company that I work for did all the paperwork for me, but the thing is, that things have gone sower since I've gotten back. My question is:

What happens if I decide to not work for them any longer? Am I no longer legal without the same employment?

If I decide to work remotely for a company back in the states, what happens when I need to renew and I am no longer working for a Spanish company?

Last but not least, if I decide to work for another Spanish company, in general, how does the renewal process work? and how many times do I need to renew before I can apply for residency?

Thanks for your help everyone!,

American, living the dream in Spain!