Would you have another interview after an informal interview if that was your first interview with the company?

I would always expect to have a number of formal interviews after the informal, or initial screening interview.

Some companies will have you go through 3+ formal interviews, while others will be shorter and others longer.

Its not impossible, but highly unlikely in today's market that you would be automatically offered a written offer after only one informal meeting.

Getting a job today is quite different from say a decade or two ago. Today, depending on the job profile that you are looking for, you may have to go through an interview process. Everything related to the job interview process depends on the post that you are looking at.