It's not a side hustle. It's probably about 20-25% of their profits.
They don’t break out the exact mixture, but we know the Google Search deal is an enormous chunk of it.
e.g. https://www.trefis.com/data/companies/AAPL/no-login-required...
For context in 2022 Adobe made $17.6 billion.
So 30% cut of iOS store is 40% more than Adobe revenue.
And I'm guessing the bandwidth + review team overhead is many times lower than, you know, actually paying software developers, marketers, sales people etc. to write and promote software.
Apple also launched the Small Business Developer program that year reducing their cut for qualified businesses down to 15% after the first year of an active subscription (it’s stupidly complicated for what it is), but at the high end, under their prior cut of 30%, Apple would have earned between $25B and $26B. If after the small developer program Apple only took home a 25% cut averaged out, then that’s about $20B. If they get 20%, then they get about $15B.
Some of that extra cut that developers get comes back to Apple in App Store ad dollars though, so best estimate with available data: between a fifth and a fourth of Apple’s 2021 services revenue of $68.42B was their cut of the App Store + App Store ads which puts it ahout on par with their Google Search money for that year ($18B).