AncestryDNA says I am 100% East Asian. Can I use my raw DNA data to find out which specific part of East Asia my ancestors come from?
Short answer: not yet.
Distant ancestry from DNA has been likened to "genetic astrology", although others consider that unfair. There's evidence to support some of the distant ancestry reports produced by genealogy companies, but there are also assumptions and statements that do not yet have enough evidence to support them.
To be able to give more accurate results, there needs to clearly verified ancestry from an area attached to each DNA sample. There are many projects that have attempted to do this. But most have sample populations which are too small, or which have ill-defined ancestry for an area (since most people, outside of isolated inbred populations, are a mixture from multiple areas).
With small sample sizes, and an imprecise way to allocate them to specific ancestry, the results will remain imprecise. It's getting better over time as more samples become available, but current assumptions are extrapolated from not enough data.
Perhaps the best way to see this is to compare different ways of calculating your "admixture" of ethnic origins. The gedmatch.com web site allows you to upload your raw results from Ancestry, 23andme, and Familytreedna, and (among other things) produce admixture reports from it.
At gedmatch.com there are a choice of admixture reports, currently MDLP, Eurogenes, dodecad, and Harappaworld (with several variants of each of these). These are the names of different research projects, using different sample populations and different methods to calculate. So they each produce different results.
Clearly, as they each produce different results, none of them are yet "correct".
So, by running each of the reports, which each have different breakdowns for areas of origin, you may get some more clues. But you have to bear in mind all of these are just estimates and assumptions from often insufficient data, so even if a result states categorically that the ancestry is "Japanese", it could simply be wrong.
In time, there may be enough samples attached to well documented ancestries to produce better results. But not yet.
If you test at Ancestry you will probably know what side of the earth you come from. You need to test with 23andme to pin it down. My daughter in law was adopted and looked very western. Her Japanese 4th cousins always asked about her American father. 23andme showed her Haplogroup is a back migration from North America and she has a small amount of Native American DNA. We did a transfer to GEDmatch and looked at her cousins Haplogroups and sent emails. We came to the conclusion she is Ainu or a mix of. If you are unsure of your origins 23andme will do most of the work, then a little detective work to lock it down. Look at your close matches and see where they came from and or write them.