Some apps offer direct APK downloads from their websites. If Google Play Services is detected, they use it for push notifications. Otherwise, they fall back to an internal background connection. WhatsApp is an example.
You still need to supply Play Services:
GrapheneOS uses Sandboxed Google Play. LineageOS requires flashing a package like MindTheGapps. There is also microG, an open-source reimplementation of Play Services APIs.
You still need to supply Play Services:
GrapheneOS uses Sandboxed Google Play. LineageOS requires flashing a package like MindTheGapps. There is also microG, an open-source reimplementation of Play Services APIs.