Travelling, I've seen a really noticeable difference even internationally before vs after COVID. Before, it wasn't unusual to encounter a lone person with music/talk bleeding through (cheap) earbuds. And you definitely did get self-consciously antisocial types treating their devices like '80s boomboxes and intentionally playing music or whatever with no earbuds, but it wasn't THAT common and usually someone would complain. You'd often find people using their devices without headphones when there was a logical reason (e.g., multiple people on this end on a call on speakerphone, or showing a group a YouTube clip), but typically volume would go off or earbuds in when the need to share was finished.
Now, it's absolutely the norm to play anything you want with volume as high as you want, no earbuds/headphones, even if it's just you. I've frequently seen couples sitting next to each other in airport waiting areas not communicating, each blasting something different on a device. Nobody has aged THAT much in two years, so I'm guessing it's mostly the effect of deterioration of unused social skills and possibly some backlash against previous social norms. The more it becomes the norm the more rude, lazy people will do it.
In the case of the child, it's not great but at least it served a purpose. Given that it's a relatively casual environment likely the parents and the other diners were thinking that the music was preferable to the child crying/screaming.
Years ago I connected my Bluetooth to someone’s phone and played Barbie girl (I don’t think you can do this now) You can if they have their hotspot turned on AND haven't passworded it. But in a crowded place I'm not sure how you'd know which device to connect to; they probably don't have their network name set to "Inconsiderate Arsehole Blasting Crackly Crap".