Like, don't check email after work hours? Or, don't be personal friends with coworkers? Or don't discuss personal plans during work small talk?
There's a lot of different ways to "separate personal and work lives", and there's a lot of different interpretations between how blurry that line can be before you have a separation or don't have separation.
At Google's scale, individuals have different preferences for all of these things. Some people don't want to read email after they leave the office at 5pm. Other people leave at 3pm to pick up kids, but sign back on from 7pm to 11pm to address stuff. Others will work whatever hours M-F but won't respond to emails on the weekend. Someone else works with a team across an ocean, and so they check email Sunday through Thursdays or Tuesday through Saturday. It's not one size fits all; it's about creating a work environment where folks feel comfortable talking about enabling flexibility without creating expectations of being always-on.
I then poke a very well chosen set of holes in the firewall: some coworkers get added on signal on my personal phone. Some personal tasks that are time sensitive happen during work time. ...sometimes pages happen during personal time.
This makes sure that for the most part, stuff stays where it's supposed to be. But when exceptions are made, they're very high leverage.