Explanation:
You can get the notes from rng2 and set them in rng1 by using setNotes.
Note that:
rng1 and rng2 needs to be of the same dimension, in this case length.
Feel free to change the ranges (rng1 & rng2) but respect the above condition.
Solution:
function myFunction(){
const ss = SpreadsheetApp.getActive();
const sh = ss.getSheetByName("Sheet1"); // change that to your sheet name
const rng1 = "A2:A" ; // range you want to add notes
const rng2 = "B2:B"; // range you want to get notes
const notes = sh.getRange(rng2).getValues();
sh.getRange(rng1).setNotes(notes);
}