where did this 2nd desktop come from, how to I move an open window back to the original?
Here’s Apple’s documentations on mission control:


https://support.apple.com/en-us/HT204100



  
  How can I move chrome back to my "main" or normal desk top where all the other screens are open?
  


Follow the directions to open Mission Control, click the desktop with the open chrome window, grab it, and move it to the desired destination desktop. 


  
  How might I have sent chrome to a second desktop to begin with?
  


Don't know. If it happens again, see if you can note your exact trackpad and keyboard gestures at the time and leave a comment.