The importance of Apple to US markets is huge. As one of the FAANGS which are 70% of the US market any large impact on its performance will create a downtrend the major US equities market.
So the Trump administration's latest move to ban TikTok (owned by Chinese company Bytedance) and WeChat (owned by Chinese tech firm Tencent 0700 HK) has some serious issues in the current form. The executive order prohibits US entities from doing any business with TikTok and WeChat or their Chinese owners.
Now virtually all consumer facing US businesses in China rely on Tencent for advertising and payments....and none is more exposed than Apple.
Approximately 70% of China's retail transactions are done through digital payments in China and of this ~50-60% is through WeChat. Furthermore the contact tracing app (required for entry in most buildings in China) also runs through wechat. So not having wechat in China literally means you cannot go anywhere, you cannot buy anything, cannot chat with anyone, cannot see all your friends social media.
China is Apple's largest market, the country generates ~17% of iPhone sales and probably a higher percentage in Appstore sales. The problem for Apple, as highlighted above, in China an iPhone without wechat is virtually useless.Therefore, in current form, and if Trump follows through and Apple abides by the law, this will literally destroy Apple's business in China and consequently have considerable impact on the markets.