No one brings up Discord? You really believe you’re talking to your friends in private? So discord pops up all those years ago and provided the service out of the kindness of their heart? For free? The terms of service is much more lawyer speak these days but when it first made it’s debut they openly admitted to recording audio and storing data. It requires your phone number for goodness sake. It’s the only app mentioned that’s actual purpose is to listen.

he's using google chrome

That answers how it happens. My guess is that he googled things relating to submarines or his browsing behavior regarding that game caused him to be grouped as potentially interested in submarines. If you are using chrome everything you do will be collected and used for marketing. No amount of extensions will save that since the browser itself is spying. If you want to minimize this happening you have to switch browser (brave is the closest to chrome) and stop using google for search.

Switching to Firefox is always a good start.

That said, using Occam's Razor, this is probably just the algorithm pushing submarine videos in general due to that other submarine accident (OceanGate/Titan) a few weeks ago, plus a bit of confirmation bias.

PS: I almost forgot that Oxenfree II was out now. I should play that.

I haven't heard of any research results that have proven that phones or apps are actually listening to people's conversations, and am only aware of a research that has proven the opposite: https://gizmodo.com/these-academics-spent-the-last-year-testing-whether-you-1826961188

If you want to avoid this happening again, this guide here as a lot of alternatives and suggestions.

Some are fairly easy (switch to Firefox) while others might be slightly harder (like maybe convincing your friends to stop with discord and use something else instead) but if you'd like to have more privacy, it's all well worth it :)

Using Google Chrome is already a bad idea. Google usually listens to your conversations from your phone through gapps.

Does your friend use social media apps on their, like Facebook or Instagram? Assuming they also have YouTube logged in on that device, it's not impossible that an app may have picked up a conversation and this resulted in a targeted recommendation.

https://support.google.com/youtube/answer/6342839?hl=en&co=GENIE.Platform%3DDesktop

https://dma.org.uk/article/is-facebook-listening-in-to-my-conversations

All good answers here, here is what I got to say:

• a lot of things could have resulted in such a thing and it is hard to give a definitive answer to your question without knowing everything I need to know, there may be and probably many privacy holes in your system and by system I mean your OS, software and your accounts settings on the internet
• maybe you both wanna take a look at a summary of discord ToS, you might wanna keep tosdr in your bookmarks it is pretty handy and an awesome resource, anyway, are you convinced now? Ready to switch to a privacy-respecting alternative? Apparently there are some privacy-respecting Discord alternatives but IDK which to recommend as idk anything about them (at 1st I was going to recommend Telegram, matrix and signal but these are not a direct Discord alternative although you can use them to chat with your friend just fine)
• switch to a privacy-respecting browser, I recommend Firefox to fight against the chromium/blink monopoly
• opt out of all types of optional tracking/telemetry in everything you have
• if you are willing to: look into setting up a system-wide ad/tracking blocker something like a PiHole or Adguard home