Confused about how to use trains/buses

I did a quick scan and it does look potentially complicated for a number of reasons. So many different routings, many asking one to make public transit connections. Take your advice from someone with local and/or firsthand experience transiting out of MXP, not expert travellers trying to figure it out from a distance.

Welcome to Italy, and good luck ;)

Where physically do I buy a bus ticket at Malpensa?

At the arrivals follow the "Bus" signs: https://images.app.goo.gl/pABZnrq7mTUgiBXZ6.

it seems I saw it best to take bus to Domodossola

The bus companies serving MXP are listed here: https://www.milanomalpensa-airport.com/en/from-to/by-coach/by-coach-other-destinations

If taking a bus, what do I choose as my destination

Since you are going from Mxp to Domodossola, choose a bus to Domodossola.

then train from there for time saver.

You must arrive at Domodossola station before 12:48 PM, to take the EC train #52 and change only once at Visp.

In Visp you will have 16 minutes to take the Regional-Express train #339 to Zermatt.

These 2 trains will take you from Domodossola to Zermatt in 2 hours and 3 minutes, transfer time in Visp included.
Add the time on the bus from the airport to Domodossola. Decide by yourself if you'd rather train al the way from MXP to Zermatt via Milano Centrale and Visp.

Everything depends on what time you clear immigration. There is a train that leaves at 11:43am from Terminal one that takes 5 hours arriving at 4:51pm. That is your best opportunity. Probably not worth a one night stay in Zermatt since you will be tired from traveling that day. Best of luck.

Thank you so much for your replies and your time. You have helped get this senior citizen one step closer to a successful journey.