How to activate and use Google Chat

Real-time collaborative chat, as exemplified by apps such as Slack, has become a common way to communicate between colleagues and friends, especially since early 2020, when so many of us transitioned to working from home. In mid-2021, Google apparently took note of this and pit itself against Slack by taking two of the features from its corporate Workplace app suite - Chat and Spaces (previously called Rooms) - and integrating them into its standard Gmail app.
Google Chat allows you to hold chat sessions with friends and groups of friends either on an informal or formal basis. The former can be done just between two people or between several in what Google calls a group conversation. The latter is done via a separate area that Google...