You can change the number you're sending to in Messages for OS X simply by clicking on the recipient's name. A drop down menu will let you choose that contact's other numbers.
One of the most highly touted features of Apple's flagship phone, Face ID, is made possible by the iPhone X's new TrueDepth front-facing camera system. It senses features by projecting infrared dots in a known pattern and measuring the distance of each, thereby creating a topographical map of the subject. Rather than using this technology only for security, Apple had some fun with it and created Animoji.
The Messages app on your iPhone can quickly grow to be one of your biggest storage hogs, especially if your friends use it to share a lot of photos with you. If after taking a look at Settings -> General -> iPhone Storage you decide you want to start deleting message threads to free up storage, you probably will want to keep some of the images in those threads.
While you would always get your messages on all of your iOS devices, a great annoyance was that if you deleted one on one device, it would not be deleted on your others. Syncing Messages to iCloud not only solves this, but it saves space on your device as well by storing all but your most recent threads in your Cloud. To activate it, head to Settings -> Messages and toggle on Messages on iCloud. When the "Sync Now" button turns blue (it could take a few moments), tap it to sync your messages.
Bitmoji is an app that allows you to create "your own personal emoji" and use it in a variety of messaging apps. Bitmoji was originally owned by Bitstrips, which was acquired by Snap Inc. (parent of Snapchat) in mid 2016. While it was quickly optimized and integrated with Snapchat, it can be used with other messaging apps, including iMessage. Here's how to install the Bitmoji keyboard on iPhone and use it in iMessage:
Adding the Bitmoji keyboard