Fundamentally the problem is that SMS is rather dated and doesn't support a lot of features expected of a modern messaging app. Apple decided to do what Apple does and made their own proprietary protocol that runs parallel to SMS. When you send a "text message" on iMessage it checks if the person you're talking to is also using iMessage and if so sends the message via Apples private service. If they aren't using iMessage it dumbs things down and send it via SMS as a fallback.

Google came along and more or less did the same thing but made their protocol (RCS) licensable which makes them slighty better than Apple, but it's still not as good as an actual open standard.

Signal is yet another solution, but they were primarily focused on security and encryption rather than new features, but fundamentally they did the same thing as iMessage initially. About a year or two ago Signal dropped the option to fallback to SMS so now you can only send Signal messages between Signal users. Unlike Apple or Google, signals protocol is open, but Signal itself is closed source and I don't believe they allow interop with their service so I'm not sure their protocol being open actually does much good.

Basically everyone sucks in their own way, but if you want SMS interop then the least bad option is RCS currently.