This web part lists all the public teams the current user is not yet a member of. They can then join any of those teams by clicking the button right next to the team name. This web part can also be added to Teams as a tab (built with the 1.7.1 plusbeta/preview version).
Open this sample in the gallery (popup panel): Open in gallery
Source: https://github.com/pnp/sp-dev-fx-webparts/tree/main/samples/js-public-unjoined-teams