When logging into hotmail on the mail app, do all contacts save into the contact list on the iPhone? Or only certain ones? If so how?
It depends on how you have the account setup in your iPhone Mail settings.
iPhone, iPad, or iPod Touch Tap Settings, and then tap Mail, Contacts, Calendar.
Tap Add Account in the Accounts page.
Enter your Outlook.com address and password.
Select the fields that you want to sync. Tap Save.
That last step is where you decide whether you want to sync contacts, calendars, etc or just mail. If you choose to sync contacts, it syncs all contacts. There is no way to selectively sync contacts.