The default messaging app is the app a phone uses to send and receive texts. RCS only works when that app supports it: Google Messages on Android, and Messages on iPhone since iOS 18.1.
Why the default matters for RCS
SMS is handled by the carrier network, so any messaging app can send it. RCS is handled by an app registered with the RCS platform, so the app has to support the protocol and be the phone's default. On Android that app is Google Messages, which most manufacturers, Samsung included, now ship as the default; the best RCS app for Android page explains the alternatives and why there are few. A customer who sets a third-party SMS app as default silently loses RCS and receives everything as SMS.
iPhone
Apple added RCS to its own Messages app in iOS 18.1, with the carrier deciding when to enable it. There is no other messaging app to choose on iPhone, so the question there is the iOS version and the carrier, not the app.
What a business sender should do
Nothing per customer. The capability check before each send discovers whether the handset's default app is RCS-registered and drops to SMS if not. What a sender can do is send a test message to its own phone and see the branded thread the way customers will.