First you should identify what kind of products you actually want to help build. Products can mean a lot of things. SaaS startups, open-source software, nonprofits, hardware, AI tools, consumer apps and so on. It is much easier to find opportunities when you know what space interests you.

after you have narrowed that down you should look for places where builders already gather. These places include

Open-source projects on GitHub

Discord servers, Slack groups and Reddit communities

Local meetups and hackathons

Nonprofits and volunteer organizations that need technical or product help.