Google Ads Bid Adjustments allow you to exhibit your advertise with more or less frequency, depending on who, when, how and where your searches are being made! Basically, they will increase or decrease your keyword's bid on each search. You can dig deeper on Bid Adjustments in this other article.
However, depending on the Bid Strategy your campaigns use, those adjustments can be ignored. On some Automatic Strategies, for example, Google's algorithm determine the Bid on each search-term, and you loose control over those variations.
On the table bellow, you can see which Bid Adjustments can be used on each strategy - even Automatic ones!
Click here to open the chart
As it shows, you can just use a -100% adjust on Device Bid Adjustments on the Automatic Strategies Target ROAS, Maximize Conversions, Maximize Conversion Value and Target Impression Share.
Also, the Bid Adjustment made on Devices for Target CPA is on top of your the Target CPA you set before.
Yet, Bid Adjustments for Audience on Target CPA and Target ROAS work prioritizing your ad's impression based on each public's bid. So, the adjustments don't actually change the bids.