It looks like React Native (and therefore Expo) does not have built-in support for SSL client certificates. There is a feature request here: Support TLS client certificate authentication in fetch API implementation | Voters | React Native
In order to implement this, I think you would need to eject to the bare workflow, then write some native code to do your HTTPS requests using client certificates and call into that native code from your JavaScript code. I think that’s what is being suggested in this comment.
Also, matson51 seems to be a bot.