Because you can only apply for the STV, Special Tourist Visa, in your home country. You can get a Single Entry Tourist Visa in Cambodia. Two different visas.
You can fly to countries but "currently" land borders are closed to tourists. Some people are posting certain land borders are open but when you read the thread, it's usually only open to local traffic.
We've done numerous visa extensions in Chiang Mai and have never had them keep our passports. Check and see if you can make an appointment for a regular extension. If not just go earlier in the morning, depending on how busy they are it should be quite quick. Just have all the copies and forms completed.
also Canadian. We applied for the original 90 day visa within Canada because I would be a trailing spouse, so a bit different. We only needed insurance for the first 90 days. We weren't asked for it when we did our extension in Chiang Mai. If you don't have a trailing spouse then what
You can fly to Cambodia, Phnom Penh. You must apply and be approved for a tourist visa which you can apply for online, it's 30 days for $30 US. Need a negative PCR test 72 hours prior with a wet signature/stamp. You will get an ATK on arrival, if negative can continue on to travel. If positive get a PCR test, if still positive then quarantine. You can apply for a tourist visa at the Thai Embassy in Phnom Penh not sure how long, but any it's I've read suggest a week at least.
Vietnam and Laos are only open to tour groups so far. Do a search in this group
join the group James linked and search some of the requirements. There are specifics for Cambodia, especially the standards for the PCR test. As of yesterday VOA's are not offered but you can get a Tourist Visa online.
yes, aware of that as mentioned in my response. The point is it's possible, might not be the type of travel you prefer but it is open under certain conditions.