What's Blue vs Green bubbles?
Disclaimer: This is a North American problem only.

Image Source: Wall Street Journal
After researching about this topic for a long time, I've realized that this is what happens when an excellent marketing team finds success. For those who live outside the US, this is the premise you're missing.
Back in the early 2000s, the world started to feel the need for a better and more powerful messaging service because the existing system only had a few functionalities. So a system called the RCS (Rich Communication Suite) was established in 2008 which extends the features of normal SMS. Apple on the other hand, developed their own service called the iMessage in 2011 which also had a ton of extra features. But in classic Apple fashion, restricted it to Apple devices only. Lucky for them, iPhones got really popular in North America.
So if you and your friend own iPhones (blue bubbles), the texting experience is amazing! But the moment you text an Android (green bubbles), the experience is trash.
So we just switch to a universal messaging app that has these features and forget about the problem right?
Turns out, the general public just use the default for Everything. The default messaging app, the default browser, the default search engine, etc. And Apple very very conveniently has integrated iMessage into the default messaging app. So the average iPhone user will never realize that they actually are using a third party service and not the Global standard. To try and convince the iMessage user to try the other services is like convincing your grandmother to try a new cuisine. We both know how that goes.
The obvious question now is if RCS is a global standard, why isn't iPhone using it instead of the age old SMS? The simple answer is, to hold their dominance and exclusivity of owning an iPhone. But due to the pressure from other companies and the government, Apple has agreed to use RCS starting from 2024. This means that for the first time, all smartphone users will be able to see typing indicators, read receipts, share high quality media and most importantly, end to end encryption. The iMessage will still continue to dominate the iPhone texting space but it is good to see that we now will have a better texting experience between different platforms.
If you enjoyed reading this, there is something that a company called "Nothing" did to compete with Apple's iMessage and it's a very interesting approach to explore. Here's the link into the rabbit hole.