FaceID or TouchID 's information is truely own by IPhone,Wechat use camera can only take photo or video,but when u login, speak six number to wechat,this is own by Wechat.

Both Alipay and Wechat Pay have those fancy terminals now in coffee shops, where you need to smile to the camera to pay. No iPhone involved. To set it up they use the same database as for the account verification.

Can you use a password instead?

Yes, but if you get deemed "malicious" it will be required: https://www.bbc.com/news/blogs-china-blog-48552907