Microsoft forced OEMs to install Windows at the time. That's a slightly different situation. As far as I know Google is not force-feeding Android to anyone.

How did they force them? With a gun or something?

Google will threaten to cancel your Play Services license if you ship services (that have nothing to do with android) competing with them. Classic case of harming competition by product tying IMHO.

They forced them like this: If you're installing Windows on all your boxes, it costs you $X per box. If you install Windows on only some of your boxes, it costs 2X or 3X per box.

Your options are to install Windows on every box, to install it on none (not really an option for big OEMs), or to be at a cost disadvantage to your competitors. In practice, it was "install Windows or lose a big chunk of your business".

That sounds like a volume discount, which is actually encouraged under most capitalist models to capture consumer surplus. Its not exactly what happened. To put it simply, Microsoft used a complicated pricing model where the OEM paid a lump-sum to Microsoft based on expected sales and Microsoft applied zero marginal pricing so every copy of windows was "free". But if they exceeded their sales they then incurred a fee for every additional unit sold. If they increased their sales, then the lump-sum would have to be renegotiated for the next contract. They wanted to tax the 'output' of the OEMs instead of just selling them at volume. It was a form of predatory pricing which got them into trouble. I don't believe competing OSes were ever a big threat to Microsoft.

No. They gave a better deal to OEMs who used Windows exclusively

In what way did Microsoft force OEMs that Google doesn't?

Could you fork Windows at the time? No. You can fork Android anytime now. That's the difference.

You can fork AOSP, which I'm sure can't run any of the top 10 apps of the Play Store unless you install Google Services, which are closed source and require a licence from Google (which in turn requires you to bundle Chrome, make Google Search the default, etc).

AOSP by itself is pretty useless because it can't run any of the apps the public is interested in.

*Can't run many of the apps. Lots of apps will run with limited functality. Chrome generally runs on AOSP, but it won't update and isn't nearly as capable as it would be with Google Services installed.

You can't freely fork the Android that's dominating the market: Android with Google services in it, key components of it are proprietary. You can't even sell some devices with it and others with an AOSP fork.