Thailand actually has a great muslim community (about 5% of the population), so the idea of halal food, known locally as ahaan Muslim (อาหาร มุสลิม), literally "Muslim food", is well understood and will not be considered rude to ask. The Muslims of Thailand are very concentrated in the south, near the border with Malaysia, but Bangkok also has an important community.
That said, no, the average Thai restaurant is not going to be strictly halal, and Thais love their pork, so you should definitely check what you are eating. As in other parts of Southeast Asia, halal restaurants tend to say in their signage, with green banners, crescent moon and star logos and the word "halal" in Arabic. This can be a useful starting point: https://www.halaltrip.com/other/blog/a-muslim-traveler-s-street-food-guide-to-bangkok/
Domestic flights in Thailand are quite short (Bangkok anywhere in the country is less than two hours), so food is generally not served. You can definitely book halal meals on longer international flights, otherwise you cannot count on them to be available.