(PS By looking at their respective GitHub pages, Blockada seems more actively developed though)
Many of these use the VPN subsystem of Android, but are not actually VPNs -- that is, they aren't routing anything off of your device. Using the VPN subsystem is just a way to be able to filter network traffic without having to have rooted your device.
The only downside is that you can't use such an app and a real VPN at the same time.
They also have some cool features like being able to send tabs between different devices.
The main reason that you'd use Safari instead of a different browser is that it has better support for newer standards. Every other iOS browser is powered by an iOS webview, which Apple lets lag behind a bit.