You know I that may seem offensive to a Canadian (PS: I'm saying this cause I am Canadian and I found this offensive!) (PPS: I'm pretty sure it's British that sound like they're saying abot when they say about.....)
Canadians, Minnesotans, and Northern Brits do. Brits make that sound in the word "cot" and "sorry" but not "about"; I am a native Pacific Northwest American English speaker. I make that sound in "about", "sorry", and words like "cot" whenever I am in Canada, most Americans from regions that don't have that sound cannot make it though, and pronounce about "aboot" when teasing Canadians. I've been trained in phonetics though, so I can make that sound.
Canadians and Brits both have that sound, so do Minnesotans. Brits Canadians, and Minnesotans make that sound in the words "dog" "cot" and "sorry"
Only Canadians and Minnesotans make that sound in "about" "out" and "house"
Most Americans cannot make that sound so when they copy Canadian accents they make an /u/ (like in boot) sound instead. I make that sound whenever I am in Canada to blend in, because I am a linguist, and I can.