Interesting questions, and something I used to take extremely personally - because when I joined Monzo, Android was ~18 months behind iOS!
Teams at Monzo will usually have the same number of iOS/Android engineers per mobile platform. Often one, though some have a couple of each. That helps us keep roughly at parity, depending on the individual engineers and the area they’re working on (e.g. if it’s old on one platform and needs investment). Though if someone takes a long holiday there can be some catching up to do 
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The new iOS health team should help set up the other iOS engineers in the company a bit more for success, hopefully enabling them each to go faster - partly because they’re directly taking away issues that a product engineer might need to go investigate, partly because they’re making things better so less goes wrong. I’m super excited about the initiative!
I think this happens best organically. For example, something I’m keenly aware of is how different search is on Android and iOS. We know that search on both platforms falls short of what it could be (I dream of a universal search across everything! To be clear tho, noone is currently working on this sadly) - when we invest in making it great, we’ll do it with both platforms at once and the disparity there will naturally go away.