Each software team organizes its communications in its own specific way. Some use Slack, Trello, or GitHub; others just sit together in the same room. There are many methods and tools. I believe it's possible to rank them by the amount of damage they cause to your project. This is the list of all of them I'm aware of at the moment.
The damage I'm talking about is caused mostly by the distance between these communication channels and project artifacts. The farther away people stay from documents, the bigger the risk of losing information. And lost information is the first source of trouble in any project.
Here is the list; it starts with the most damaging communication means and goes down to the most mature and professional ones, which cause the least amount of trouble:
Coffee Breaks. This is the most dangerous thing—you can never keep track of them, you won't know what they were about, and there is no "search" button anywhere. Everything you say standing next to that coffee machine will be lost. Nothing will be converted to project artifacts.
Phone Calls. A bit better than coffee breaks but still a big issue. Phone calls are completely untraceable. Information you exchange on those calls is gone forever. Well, you can record them, but searching through phone call records is a tough task that nobody will do, ever.
Meetings. This is the next step after coffee breaks, because there is some structure and minutes. Meetings can be recorded (both on and offline), with their results filed somewhere and decisions documented. In reality, none of that will actually happen. Meetings will just kill your time and your sponsor's money.
Emails. If you can put some formality into emails and discipline all participants, your email history may be considered a project artifact in itself. How organized and easily browseable will that artifact be? That's a good question. In most cases, it will just be a mess.
Mailing Lists. They are better than emails, because some software is archiving them and making them available and browseable. But it will be difficult to find where exactly what topic was discussed, where decisions were made and why, who suggested what, etc.
Slack. There are many similar alternatives that are basically online chats. The main problem with all of them is that it's difficult to categorize such a chat, group messages together, or find something later. It's merely a flow of information that becomes useless just a few days after. Of course, if you really want to find something there, it's possible. But the quality of such a "document" is very low.
Trello. By Trello I mean any task/ticket tracking system—they are great instruments to immediately turn conversations and discussions into project artifacts. You don't need to document anything; it's already there. The problem is that they are still rather far away from the main project artifact: the source code with its commits, merge conflicts, build logs, etc.
GitHub. This is the best instrument you can use. It integrates communications with the product itself. The code you write and the discussions you have around it are literally in the same place.
Which one of these is your project using right now? I would strongly recommend you stay away from communication channels at the top of this list.