When searching/adding addresses in Mail, it primarily looks in the Contacts app. It will also show addresses from previously sent or received emails when you do a search.
Mail can send to groups. Which is surprising. You didn’t used to be able to, and I’m not sure when they added it. While you can have groups in the Contacts app, for some strange reason you can’t create groups the native Contacts app. Nor can you add contacts to existing groups.
There are three ways that I know of to send group emails.
One is to go to the icloud.com site and use the tools in the Contacts app there to create groups and add contacts to them. You have to be using iCloud to store/sync your contacts.
The second is to use a third party contacts app that syncs with the native Contacts app. The one I use is Cardhop:
‎Cardhop
Once a group is set up you can search for and add it to an email just like any other address. In the case of multiple addresses in a contact, it appears to choose the top/first one listed. There is the ability to tap on a contact in Mail after adding it and switching the address.
A third choice is an old hack that used to be the only way to send group emails in the Mail app. Use a third party app that creates a special contact entry in Contacts that has multiple emails crammed together. I use to use Mailshot for this.
‎MailShot- Group Email