First off, if you’re concerned about phone privacy, consider a custom OS for your phone that respects privacy such as GrapheneOS.
It’s easy to figure out that your device isn’t listening to a constant audio stream 24/7, since that would drain battery and send a lot of noticeable data over the network. However, it is entirely possible to listen for certain keywords as you mentioned, and send them encrypted with another seemingly legitimate packet. There’s no way to be 100% certain, but it is possible in theory without draining too much battery.
The steps you took are good, making sure that apps don’t have any permissions they don’t need. Privacy is a spectrum, so it’s not “all or nothing”. As I mentioned before, if you’re seriously concerned about mobile privacy and want a solution, you can get a custom operating system that can remove any privacy invasive elements. GrapheneOS also allows you to disable the camera and microphone system-wide (although this functionality is present on some other Android builds).
If it eases you any, a lot of these advertisements happen to be coincidence and trigger confirmation bias. It could be that those ads happened to show up by coincidence, or that advertisers managed predicted your interests, or that you got tracked by some other means while downloading the movie. The possibilities are nearly endless.
If anything, I love GrapheneOS for its “Network” permission toggle. It’s nice knowing that my keyboard (or any other unnecessary apps) can’t phone home.