If I have insurance on my phone through T-Mobile but want to cancel my plan can I get Apple care straight through apple or I won’t be able to have my phone insured anymore once I leave the carrier

You would only be able to get AppleCare coverage if you purchased the phone within the last 60 days. See below from https://www.apple.com/support/products/iphone/

How to buy

1. Purchase either AppleCare+ product with your new iPhone.
2. Or buy it within 60 days of your iPhone purchase:

• • On your device (go to Settings > General > About and choose AppleCare+ Coverage Available)
  • Online (requires you to verify your serial number and run a remote diagnostic)
  • At an Apple Store (inspection of iPhone and proof of purchase are required)
  • By calling 800-275-2273 (requires you to run a remote diagnostic and provide proof of purchase)