Plain white T-shirts are undershirts.
Unless I am working in the backyard, or in the garage, I don’t like being seen in public in a plain white T.
YMMV
I detest the use of undergarments as an outer garment. I don't even use white t-shirts for work around the house. There is a Midwest builders store named Menards that will have sales of t-shirts with their name on them and you can purchase them for two or three dollars, sometimes free with a rebate.
When I am back home in Michigan I purchase those and use them for my work shirts. They are cheaper and not underwear.