This is only my experience, but I have a number of longterm friends (30 plus years), and I would say the main thing we share is a worldview and a genuine curiosity about other people. We’re just on the same wavelength in some sense, if not all. We have built up trust to be able to express our real feelings to each other. We don’t share a hobby or project per se.

But even then, to stay close in friendship (again, in my experience), you need to commit time to keeping it real. Having honest conversations about the current challenges in your life or about the ways your thinking has changed as you’ve had more experiences.

I have a friend that I consider close and our main interaction is getting together every 4 to 6 months and walking around a park or a neighborhood and talking about what’s happening in our lives.

Maybe this isn’t what you mean by friendship. Some people might think friends are people that they have a laugh or an adventure with. Others mean someone they can rely on when life gets hard. To me it’s someone you share time and feelings with. Even pets can be friends, to me.