This is set to change, but right now iPhones only support the most basic form of text messaging with android phones. Texting between iPhones just works better. The images are higher quality, reacting to messages works right you can can reply to people, where as soon as you try to include an android number all of that drops away

gpt says

The distinction between iMessage (blue bubbles) and regular SMS/MMS (green bubbles) on iPhones matters because iMessage offers additional features like read receipts, typing indicators, higher-quality media sharing, and seamless syncing across Apple devices. While the color of the bubble itself may not be significant, the functionality and experience differ between the two, impacting communication capabilities.

Your telco/carrier can (and do) "read"* your SMS chats.

* Capture, store, share with various authorities and data services, and sometimes advertise against, as well as make available to other people on your account and carrier support.