How can someone connect to my phone without consent and view my text messages?

They cannot tap your phone. Someone with your credentials could access your iCloud and Apple Account.

See If you think your Apple Account has been compromised - Apple Support

If someone signs into your account with your password on their device, all data will be shared. You can view if another device is signed into your account by going to Settings > [name on top], and scroll to the bottom to view the devices. They cannot do it by tapping your phone.