Well, they could have participated in the same hobby groups or clubs even after high school. For example, if they were into a particular sport like basketball in high school, they might continue to play in local leagues and see each other there. This would not only keep them in touch but also strengthen their friendship as they share a common interest.
Some friends stay in touch by making plans to meet up regularly. Maybe they decided to have a monthly dinner or coffee date. They could also plan trips together during vacations. By spending time together in person, they can maintain their friendship and create new memories even after high school.
They might have kept in touch through social media. It's so convenient these days. Just a few clicks and you can share your life with your friends. Or they could have exchanged phone numbers and texted or called each other regularly.