Citizenship for Earth
Here's how I believe it goes for a child born in a foreign country and adopted by US citizens while they reside outside the US, if they file for the child's citizenship after returning to America using the N-600 process:
1: You must have documented legal custody of the child for two years
2: File both the I-130 to prove your legal relationship to the child and the I-485 to adjust their status to legal alien resident (if not already done)
3: Once those the child is accepted as a legal alien resident, you can file the N-600 requesting expedited naturalization
I mailed in Earth's I-130 and I-485 on 4-18-18 and will continue to share our citizenship process through this path here. I will also add this post to the US Citizenship page, where you can also find information about the N-600K citizenship process.