Hi Spencer696,
There are four ways to send messages on iPhone. The first is standard text messages (SMS or MMS), which are sent with your phone number and appear as green bubbles. The second is RCS, which is available in the US and UK with certain carriers. These are sent with your phone number and allow for high resolution photos, reactions, read receipts and typing indicators, appearing as green bubbles. The third is iMessage, which allows you to send messages with high resolution photos, text effects, read reciepts and more to other Apple devices. iMessage allows you to send messages to Apple devices, even without phone numbers, so you can reach someone with their email if it is linked to their Apple Account. The fourth is Messages for Business, which allows for messages to businesses, but I won’t go into detail.
When you send a message to someone with an Apple device, you will send iMessages, which appear as blue bubbles. You can go to Settings>Apps>Messages>Send & Recieve, and select whether you want to reach people with your phone number or email under “Start New Conversations From”. You can still leave your email under “You Can Recieve iMessages to And Reply From” if you want people to be able to reach you by email. If your phone number doesn’t appear here, add it to your Apple Account.