I have read the following:
Other than the above, I also spent quite some time on google and xkb documentation.
I assume, that many users like me switching from Mac to Ubuntu would like to swap the ctrl and alt keys immediately after finishing the installation.
My purpose to post this question (and answer) here is to give a quick solution instead of spending so much time going through the above threads and docs.
Also, none of the above provide the correct answer. (This is not to say that I don't appreciate the help. The above posts did ultimately help me achieve what I wanted to but the responses were not complete.)
Thread 1 only swaps left ctrl with left alt
Thread 2 swaps ctrl and win even though the question was not about win key