How does WeChat automatically register and log on to the public account after paying attention to the public account?

For example, if you follow the public account of Dangdang and enter Dangdang, the account is automatically registered and logged on. No webpage authorization for this step. Is the documentation mp. weixin. qq. comwiki? Tresourceres_main & amp; idmp1421140842 & amp; token & amp; langzh_CN .... For example, if you follow the public account of Dangdang and enter Dangdang, the account is automatically registered and logged on.
No web page authorization for this step, document https://mp.weixin.qq.com/wiki? T = resource/res_main & id = mp1421140842 & token = & lang = zh_CN.
How can this problem be achieved?

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For example, if you follow the public account of Dangdang and enter Dangdang, the account is automatically registered and logged on.
No web page authorization for this step, document https://mp.weixin.qq.com/wiki? T = resource/res_main & id = mp1421140842 & token = & lang = zh_CN.
How can this problem be achieved?

Two authorization methods are available:
1. silent authorization
Obtain the user's openid.
2. Manual authorization
You can get your openid and more information.

The first type of silent authorization is used to obtain the user's openid and check whether the current openid is bound to a website account.
Login Without binding. After login, bind the openid to this account
Automatically log on to the background after binding

Compare your openid obtained on the Web page with the openid stored in the database. In the same way, it is an authenticated user and you can directly access it without authentication.

Dangdang's public account has not been followed, but openid is obtained in three ways:
1. silent authorization
2. Manual authorization
3. When you make a request to a public account, such as a reply, click a menu, follow, or cancel a follow event, the developer's URL will be forwarded with the user's openid
If it is an implicit test, it is estimated that it is one of the first and third methods.