I’ve noticed that too. Intentionally veered a conversation into a different topic and, lo and behold, I get “relevant recommendation” short time later. That was, not entirely coincidentally, the same day I unlocked the bootloader and flashed a de-googled ROM.
Android is sending a ton of data, though, even if you’re not doing anything internet related. It, also, kinda reacts to “okay, google”, which wouldn’t really be possible if it wasn’t listening.
Now, it obviously doesn’t keep a continuous, lossless audio stream from the phone to some google server. But, it could be sending text parsed from audio locally, or just snippets of audio when the thing detects speech. Relatively normal stuff to collect for analytics purposes, actually.
Now, data like that could “easily” get “misplaced”, of course, and end up in the ad-shoveling machine… Not necessary at Google’s hands: could be any app, really. Facebook, TickTok, random free to play Candy Crush clone, etc. But if that data gets into the interwoven clusterfuck of advertisement might, it will likely end up having an effect on the ads shown to the user.