

somewhat
Yes. Only in the EU and only since 2024 when Apple was forced to do it by new laws. It’s reasonable to assume Google would be subject to the same laws.
If you live outside if the EU, it’s “no sideload for you!” There are computer programs that can do sideloading to iPhones, but they have limitations, like having to refresh the sideloaded apps every seven days.
The limitations depend on which program you’re using - there’s more than one - which is why I only gave a simple example. And if you have to pay for a function that is otherwise free to many others, that’s a limitation.
Side loading is installing an app from anywhere but the official store. So by definition “third party” is side loading. Whether it’s another store or authorised is irrelevant.