It is Nintendo directly, unfortunately. The issue of mining for electronics (eg. the ethics surrounding coltan mining, which is a material used in cell phones) can be related to forced labour, but is not the one that I believe OP is referring to.
The report by ASPI directly names Nintendo as one of 83 companies "directly or indirectly benefitting from the use of Uyghur workers outside Xinjiang through potentially abusive labour transfer programs as recently as 2019". Other notable companies named include Abercrombie & Fitch, Adidas, Apple, BMW, Calvin Klein, Dell, Gap, General Motors, Google, H&M, Mercedes-Benz, Microsoft, Nike, the North Face, Samsung, Sony, Uniqlo, and Zara.
The report's appendix further details the exact factories in question, and which factories supply which brands. For Nintendo, two companies whom they do business with are listed in the appendix - one that produces "precision parts for electronics such as backlights and battery covers", and a contract electronics manufacturer.
I never thought I'd see the day when Animal Crossing and my academic life would collide, but here we are...