Pretty much, the organ harvesting claim in particular originally was based in the falun gong belief that(Disclaimer: This is not a bit) their spiritual practices literally made their internal organs superior in quality and impervious to disease or physical ailments(This leads into their belief that modern medicine is unnecessary and a satanic communist tool) and therefore the Communist Party harvests those organs to make their party elite live longer.

This was later transplanted basically wholesale into the idea of "halal organs", the claim that there are muslim elites who refuse to receive organs from anyone who has not adhered to halal requirements in their lifestyle, and therefore the Communist Party are systematically harvesting organs from the Uighur muslim population to sell on the black market, presumably to the muslim nations that stand with China.

There has as far as I have read never been any evidence to support either claim that isnt either a claim of personal experience or a "whistleblower" statement that also is exclusively a personal claim of experience, unless China also is running a whole black market supply system of immunosuppressants and other post-transplant support medical care, you would see very obvious signs such as consumption of those medical supplies being obviously higher than recorded transplants.

The only signs like that that has been pointed to is that China has a notably short waiting list for organ transplants from what I have heard, but thats basically where the supporting evidence begins and ends, no further investigation has borne fruit.