Can someone explain to me how the 6 months max visit works when you’re a US visitor that doesn’t need a stamp. Is it 6 months from when you enter or from the last 12 months? Ex. Could you stay 6 months then leave for a week then come for another 6 months or does it count from the current days last 12 months?
It's six months from the time you entered Canada, unless CBSA only gives you a shorter visit.
Technically you can stay six months, leave for a week and then try to re-enter to get another six months. However again, it's up to CBSA if they allow you back in and whether they give you the default six month visit or only a shorter stay.
If you want to remain in Canada longer (beyond the six months), then it is a better idea to apply for an extension from within Canada without leaving vs. leaving for a short trip to the US and then trying to re-enter Canada for another six months. Applying for an extension from within Canada has a higher chance of working out if your favour.