For our iOS applications, we leverage system fonts such as Helvetica, Helvetica Neue, and Palatino. Apple provides detailed information about its system fonts at the following link: Apple System Fonts.
For our Android applications, we utilize the open-source font Roboto. Google provides an FAQ on their fonts, available here: Google Fonts FAQ.
Note that we "reference" these fonts without embedding them directly into the app, meaning there are no fonts bundled within the app's package. This approach means additional licensing is not required for your events.
On the web-based platforms, we instruct the user's browser to use specific fonts in a designated sequence, contingent on whether those fonts are installed on their system. Typically, Helvetica Neue will be used.