What is the best community app in 2021

Hi there, I am working on a project that involves building up a online community. The tool should cover the following funcions: activity feed, contact and friend management, posts and polls, calendar, notifications, clear and modern user experience, android and IOS App, web plattform. As it is a non-profit project we prefer a tool without or with low cost. I would be very grateful if you help me. Which tool woud you recommend? Thanks a lot. Fabio