I’m guessing you’re not going to like this answer, but Microsoft YaHei is NOT licensed for redistribution / commercial use. See https://learn.microsoft.com/en-us/typography/font-list/microsoft-yahei for details. Microsoft licensed it for use in Microsoft products. I can’t fathom why Esri has it as a choice in Experience Designer other than that because it is included with Office applications, it is on a lot of Windows computers. Despite having the latest version of Office 365 on my Mac, I do NOT have YaHei on my machine. So, even though it is a choice in Experience Design on my computer, choosing it just changes it to my default serif font.
If you look at the code of a published Experience Design page you will see that although the font tags call for Microsoft YaHei, there is no mention of YaHei in CSS tags (linking to webfonts / woff2 font files). This means that IF you have YaHei on whatever machine you are using to view the webpage, it will work. Otherwise it won’t (doesn’t matter if it’s iOS, macOS, Android, or even Windows).
I’ve seen a couple of instances of Beijing Founder Electronics Co. Ltd going after people who even put screenshots of Word documents on their company sites where YaHei was visible, so unless you want HaYei badly enough to license it for commercial use (seems to be about $10,000), I’d highly suggest using another simplified Chinese webfont. Personally I recommend Noto Sans simplified Chinese (Google fonts link)
You’ll need to create your own theme in Experience design, but this is how you add your own font: https://developers.arcgis.com/experience-builder/guide/theme-development/#use-custom-fonts
Good luck.
A man's feet should be planted in his country, but his eyes should survey the world. - G. Santayana