In my current remote job and in my previous ones, we use:
1. Zoom (or Hangouts, or etc., in other words, video)
2. Slack
3. Sometimes email
4. Sometimes phone
We use video and Slack quite a bit.
Both my current full-time job and in my last full-time job with a distributed team, every so often, people are flown in together to meet up, hang out, and build relationships.
So as I mentioned in one of my responses above, remote work is not intended to 100% replace in-person relationships. Video and Slack did not build the same kind of experience I had when I flew into San Francisco and spent some time strolling towards the Golden Gate Bridge park with two of the other team members while talking about life, rather than work. Not everything about building relationships with other people has to do with being productive.
I've found the idea "impromptu hallway conversations" come up so often, I suspect it is a kind of echo-chamber narrative that hasn't really been examined for what it is. Having said that, when I visited my team, the office was hosted in a co-living space. I had a lot of impromptu hallway conversations ... outside of the team with all sorts of people, perspectives, philosophies. But that is not the normal experience people have when working onsite, either.
"General social bonding" isn't always what it is cracked up to be. Most people do not allow themselves sufficient honesty or vulnerability to have real conversations. It's more the case, social interactions often has more to do with people's masks and social selves interacting.
By the same token, I have found that effective use of Slack following the Kanban principle of "Make Work Visible" allows me to communicate in a way that I can't easily do with an on-site presence.