How to transfer to the main from non-existent 3rd level domains?

Question

Web servers

Visitors often go to 3rd level domains that no longer exist.
As in a regular hosting, carefully transfer them to the main second level,
so that they do not see a non-existent page
(It is useless to remember all the subdomains that were before and create them)
ps. 3 domains are not used at all, that is, it is possible for all subdomains of the 3rd level to be automatically transferred to the main

Answer 1

In .htaccess, write something like:
ErrorDocument 404 /index.php
Then, for any 404 error, it will be redirected to the main page of your site (if it is index.php)

Answer 2

What web server do you use nginx or apache?
In nginx, I do this:
:/# cat /etc/nginx/conf.d/default.conf
server {
listen 80 default_server;
server_namelocalhost;
error_page 403 = /403.html;
location = /403.html {
root /usr/share/nginx/html;
internal;
}
location / {
deny all;
}
Server_name can be generally left empty.
Accordingly, if there is a config for the domain, then it is processed, if there is no config for the requested domain, then it sends to 403, well, or to any other page you want.