Google gives Android users a way to install unverified apps if they worked hard for it(including waiting 24 hours)
(android-developers.googleblog.com)
/e/ OS is just a degoogled Android (similar to Graphene, but not so security oriented). You can install the same apps - though some might not work properly.
Sailfish OS is Linux, but if I understand it correctly they have a compatibility layer enabling you to seamlessly install Android apps on it.