The FC-31 has been tested and refined over the past ten years as part of a private from Shenyang. It is good enough that the Chinese Navy decided to choice it as the basis for the J-35 instead of the well-established J-20 which is mass produced in the hundreds
I do not know much about the AMCA but I know it is not flying or tested or was in a competition with the national 5th Gen fighter for an important position.
To understand how important, the PLAN has given its landbased units (hundreds of fighters and bombers) to focus on its carrier arm. The J-35 is the centerpiece and it had to beat out a successful advanced fighter already in mass production with the PLAAF.