You can also use a cross-platform message app (WhatsApp, Signal, etc). It requires everyone to install whichever app you choose, but it's better than only being able to communicate with people using the same platform.
SMS and MMS are open standards supported out of the box by all phones, but are more limited than internet-based chat apps. RCS is a standard Google have been trying to promote, but Apple refuse to support it (because it would compete with iMessage, and if it were cross-platform would remove one of the impediments to iPhone owners switching) and so it suffers the same problem as iMessage only in reverse. So it really comes down to either using SMS/MMS or using a third-party chat app.