Why is this happening?
This is because the OFFICIAL CDN of WordPress blocks some Chinese IP. How to solve it?
1. Download the offline installation package from various channels
2. Functions.php code points to the domestic mirror installation package
The code is from WS234.com

//WordPress Custom upgrade packages
add_filter('site_transient_update_core', function($value){
    foreach ($value->updates as &$update) {
        if($update->locale == 'zh_CN'){
            $update->download   = 'https://ws234.com/img/wordpress-5.3.1-zh_CN.zip';
            $update->packages->full = 'https://ws234.com/img/wordpress-5.3.1-zh_CN.zip';
        }
    }
    return $value;
});

3. Use plug-in WP-China-Yes
“The Real Bad Guy” has set up a cache acceleration node in Mainland China to speed up the access of WordPress official service in Mainland China and avoid 429 error. This node can accelerate the “plugins and theme stores” you see in the background of the WordPress website, including the download of theme plug-ins and the loading of images in the mall, etc.
In order to enable more WordPress users to use the Mainland acceleration node, “The Real Bad Guy” has developed the WP-China-Yes plug-in, in order to help you easily and succinctly replace the service links.
Reference source:
https://www.wpdaxue.com/wordpress-org-429-too-many-requests.html#comment-30578
https://www.ibadboy.net/archives/3204.html
https://github.com/sunxiyuan/wp-china-yes
4. Use the plug-in kill 429
Lightwave recently developed a plugin, Kill 429, which solves the problem of 429 error reporting and quickly installs WordPress versions, themes and plugin updates by optimizing the network of the server in China to access the WordPress data server. The plug-in comes with its own circuit, which can be modified to its own circuit.
But it seems that the current built-in circuit is not very good
Reference source
https://www.wbolt.com/plugins/kill-429?invite=2363
https://static.wpdaxue.com/img/2013/08/kill-429.1.0.0.zip