I've noticed that people have been able to edit some units' and planets' encyclopedia explanations. What XML file/s are they located in?

They are in mastertextfile.dat in the text folder in Data.  You needthe dat editor (from the tools and tutorials section) to edit them.

but to know which ones to edit u need to look at the thing u want to edit's XML file and look for  TEXT_unitname_text or something similar .. its near the beginning of the unit.. its like or something like that.

It is easy. It is referenced as Text_Tooltip in data file.