So the way compute used to work, is you could install any program you want from anywhere. You could buy a program from a web site or copy a disk and install the program.
Smartphones have been around since the late 1990s in various forms, it used to be, you could just install whatever you want.
Then, in 2008, Apple released the iPhone app store, and it was a closed space, a "walled garden". You can only install apps on their phone if they approve them.
Google decided to join the phone race and released a phone where one could still install applications from anywhere, not just their store. There are multiple stores like others have mentioned, or you can download an APK file from anywhere and install it on your phone.
Part of their behavior since is slightly open to interpretation, as the technology is now used by everyone, not just tech nerds. People could install "bad" programs, and they could lose money, cell networks could be compromised, etc.
It likely costs a lot of companies a lot of money to deal with dumb users doing stupid shit. So from one perspective, making it extremely hard to install unknown programs from anywhere will curb that expense.
It could be a defensive move, as LLMs now allow anyone to write computer software with very little knowledge of it, and it is just bad timing.
On the other hand, since the beginning of computers, the owner of the machine could run whatever software they wanted.
This move by Google is basically making it so there is NO mobile compute platform that the owner of the device actually owns, and is allowed to do with their hardware what they want. Apple or Google, that is it. Apple had always been closed, which should have been made illegal, but I digress.
It has been a slippery slope with Android for almost 2 decades, and this move is basically the end of the ability for free humans to install free software from anywhere on the hardware they own and paid anywhere up to $3000 for.
Basically a huge dive for personal freedom on a planetary scale, decided by one corporation.
It will be once the bubble pops. Small local tuned models for specific tasks that the user powers are much less expensive for the tech companies than tech companies powering and watering datacenters.
Right now the tech bros genuinely think people will be cool paying hundreds of dollars a month to rent a GPU for all their Internet tasks. AI fatigue is already setting in.
The tech bros' investors will pull funding once they realize how asinine that is long-term. Probably already starting to, with the likes of Zuck trying to use green charity money to fund his LLMs.