For millenials in the USA, that's SMS/text messages. I'm a older than millenial, but too young to be a boomer. I prefer email. Boomers I know tend to prefer phone calls. "Why can't you just pick up the phone" is a common complaint.
Not really. My family and other Spanish circles largely use WhatsApp. That absolutely did not exist when things got started; heck, I remember when my dad got his first cellphone, which definitely couldn't do anything but SMS. Discord is also much more recent and a big subset of my generation has had no trouble embracing it; it is probably the messaging app I use most these days, by message volume. Way back we'd have been using stuff like MSN Messenger. And since moving to Japan I've seen things shift from Skype to LINE and now slowly to Discord.
Voice calls are still popular in my family, but they have no trouble embracing different apps and ecosystems for it; I recently got my immediate family to switch to Telegram from Hangouts for talking among us, since Google botched their messaging ecosystem again (I refuse to use WhatsApp because it's by Meta but that's a me thing; either way Telegram is also an increasingly popular alternative overall).
Americans being stuck with SMS when superior alternatives have existed for ages is something unique. Then again, so much of communications infrastructure/culture in America is bizarre (Paying for incoming phone calls on cellphones? What? That one shocked me when I spent 3 years studying in the US.)
Japan is interesting because their feature phones used internet e-mail instead of SMS, but I don't ever recall seeing a client that threaded e-mail conversations like SMS clients ended up doing.
There is a generation in-between millennials and boomers, it’s called Generation X. It refers to people born in the 70s and early 80s.