And you can usually tell the difference between a tailored suit and something ill-fitting.

I find that buttoned shirts only work if your belly is flat, so you can wear it in your pants.

With even a tiny bit of belly fat, ime it is better to just wear a t-shirt and not wear it in your pants.

I don't think this is true, but you won't be able to just grab a shirt off the rack and rock it. Look at Penn Jillette when he was larger, as an example. He was always dressed to the nines. He also strategically incorporated vests into his wardrobe too.

Depending on your shape, a simple undershirt might be slimming enough, or adding shirt stays or shirt garters might help. Worst case you will have to get it tailored. A tshirt is obviously cheaper and easier though, but that signals something.

When I was a fair bit heavier though very active, off the rack shirts didn’t really fit me very well. Tall upper body and very broad shoulders from sports didn’t help. These days I seem much closer to just being able to buy stuff. Bought a new blazer last year and it didn’t need any tailoring which never used to be the case. And shirts work well enough.

Wear braces under your suit, and shirt garters. The combination keeps your pants and shirt exactly where you want them.

Thanks. It looks uncomfortable though and it seriously makes me wonder if there isn't a better way of keeping a shirt in place.

Shirt garters aren’t uncomfortable. I wear them with suits all the time. The other way to do it is to never lift your arms over your head or bend over at all.

Tailors can make a button down shirt that will fit your body. You will look fat, but you can look much nicer. It will cost you though.