HERE'S HOW GREEN BUBBLES ARE GETTING UPGRADED IN IOS

Summary from the TLDR newsletter

iOS 18 introduces RCS support for messaging, enabling chats between iPhone and Android users to have many features core to other messaging apps. Cross-platform chats will now support high-resolution media, typing indicators, read receipts, and more reliable group messaging. Not all carriers support RCS on iPhone yet. Apple's RCS support doesn't include cross-platform end-to-encryption between iOS and Android.