When an android phone connects to a wifi network a check is done if internet is usable by that network. It checks as well if there’s a paywall installed.
For that it requests a special webservice and expects a http result code of 204. This service is provided by several google servers (connectivitycheck.gstatic.com for http and www.google.com for https). It is called “captive portal check” and implemented in com.android.server.connectivity.NetworkMonitor.
Even LineageOS (base of /e/ as i understand) does so. If /e/ is really degoogled it means that either these checkes are disabled or the domains are changed and the service is be provided elsewhere.
How is it done?