It's important to note that competition law is broader and more broadly enforced than antitrust law in the US. A particularly relevant area for these issues is "Section 46 - Misuse of market power". This is distinct from laws in relation to price fixing, monopolies and merges. The ACCC (and possibly the later, Australian courts) will be trying to answer if Google / Apple are misusing market power as part of this inquiry.

One area that is particularly ripe is "the market for in-app payments". The ACCC would be looking whether it can establish whether under s46: a) does the company have substantial market power? and b) is it engaging in conduct for the purpose, effect or likely effect of substantially lessening competition? More info is available here: https://www.accc.gov.au/business/anti-competitive-behaviour/...

s46 - Misuse of market power was revised for easier enforcement/litigate under in 2017, and you can see a list of active cases include a case from Epic Games vs Apple and Unlockd Ltd v Google (Filed in Australia): https://www.lexology.com/library/detail.aspx?g=34d307e2-1d63... It is a BIG deal for a private company to take action under s46. There used to only be a couple cases every decade.

It is possible that an undertaking or court-order that could emerge out of this inquiry would require Google/Apple to make it easier for apps to accept in-app payments from providers other than Google and Apple in Australia.

As far as I know, the USA does not have an equivalent area of law, and if it does, it certainly is not enforced.

*disclosure: I worked at the ACCC 2008-2011 and these are opinions of my own.