To answer your question, MAC/NTM needs to be inhaled in substantial quantity to cause an infection - such as in the vapor from a shower. The amount of NTM released by an infected person is not considered contagious.
Here is a pamphlet about the infection:
https://ntminfo.org/wp-content/uploads/2023/12/NTM-INSIGHT-Pamphlet_AA.pdf
I can tell you from personal experience that even though my partner and I both have Bronchiectasis, they never caught MAC from me in the 4-5 years I had it, before I was treated and during treatment. Believe me, I was concerned about it too, but both the pulmonologist and infectious disease doc assured us it would not spread, even though we shared a bed and bathroom. There are some other opportunistic infections, like Pseudomonas, that can possibly be spread by close contact.