Most people on the Apple Ecosystem use the iMessage App on iPhone, iPad and Watch OS. They use the Messages App on the Mac. Yes, there are other Apps you can use for texting, but the Messages App is by far most used. This article explains how iMessage and Messages work and also how to troubleshoot any issues you are having with either. I have tried some third party Apps that are not too bad, but nothing compared to the feature set and ease of use of iMessage. I learned some troubleshooting steps from reading the article, I am sure you will too.