tls: install a PFX / SSL certificate downloaded from the server on an Android / iOS device in a React Native application?

I am creating a React Native application that downloads an SSL certificate file or a PFX from the server or a remote file storage. After obtaining this file, I want to install this certificate on the device so that only my application can access it. I want to use this certificate to facilitate secure API calls to another server that speaks HTTPS.
I guess I should use the keychain in iOS and the keystore in Android to store the certificate, but I'm not sure if it allows me to store a PFX. And after storing it, how do I use it for API calls that I make later?