There are only 3 types of bids, that we have in Apple Search Ads, but it's very easy to get confused by it and how it works. At the Ad group level, there are 2 bids - CPA Goal and Default Max CPT Bid, and at the keyword level, there is CPT Bid. Default Max CPT Bid and CPT Bid are almost visually identical, but they have nothing in common.
CPA Goal is your target Cost per Install, that you want to have as a result. It could take more time to reach the results of install volumes if you are using CPA Goal, but the result is totally worth it - it's possible to acquire installs x2-6 times cheaper using this bid (depending on the country and app category).
Default Max CPT Bid is a bid at the Ad Group level, which works only if you do not have any keywords in your Ad Group, for example in a Discovery campaign.
CPT Bid is a bid at the keyword level. You need to scale it, if you are working with SKAG campaigns, or any other type of campaign, that have keywords in it.
To sum up, there are 2 key ideas of this article:
Give CPA Goal a try, the result most likely will be amazing
If you are working with SKAG campaign, you need to scale CPT Bid at the Keyword level and you can totally ignore Default Max CPT Bid at the Ad group level. If you are working with Discovery campaign, you need to scale Default Max CPT Bid.