> Are US letter sizes what people use instead of A4 in a workplace for documents and such (seems confusing if so)

Yes, it is just our standard like A4 is yours. When you pull a paper out of the pack it is A4 when we pull it out it is ANSI A, commonly known a US Letter size. Instead of 8.27”x11.69”, we use 8.5”x11”. We also commonly use US Legal size, which is 8.5”x14”. Slightly longer and can fit in the same envelope.

> do printers sold in the US default to US letter sizes when printing?

Yes. However all of our printers can do all sizes since our paper is slightly larger, while an A4 specific printer couldn’t print a US letter.

I don't think I ever had a A4 printer that could not also print Letter. It is almost certainly the exact same models that they also sell in the US, but with just some bit flipped in the firmware to change the default paper size.

My current printer, like most printers I believe, can be configured to print custom sizes. The maximum is something like 9 or 9.5 inches wide IIRC, and the length can be set much longer.

Except pretty much all printer drivers these days can down convert from letter to A4.

Margins on left/right might be skinnier, but length wise US letter fits.