ABCD ancestry measures (genetic_af_*)

Hi,
Does anyone know where to find the information on how the ancestry measures were derived? (i.e. genetic_af_african, genetic_af_european, etc)
I’ve checked the release notes (v2 and v3) and the data dictionary, but I only found short definitions of the field, e.g. “Proportion of European ancestry” (https://nda.nih.gov/data_structure.html?short_name=acspsw03).
I would be grateful to hear if someone has any pointers to more detailed info on these variables!
Thanks in advance,
amaia

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