javacript can be served to browsers that comes from server. but we can also write javascript code in the browser console. so my question is how does a browser know that javascript is written by end user or it has come through webpage. is there some sort of ownership mechanism or stack implementation?

can i perform xss attack from brower console. if not then why?

i have just encountered a situation where a function was not defined inside a webpage script so throwing error. then i defined that function in browser console and then error changed to something else. (the whole code was written by me, website was mine too. but on a proper domain and is in production)