Where can I download ZBT-WG3526 toolchain and setup the development environment. In order to add openVPN and mwan3 packages

Just purchased a ZBT-WG3526 ( MT7621A) device recently and want to add the openVPN package and multi-wan (Mwan3) in the device by myself: I'm very appreciated if any expert can help the following issues:

  1. How to install the development environment ? or I can download the develop VM and develop my application directly?
  2. Where can I download the toolchain?
  3. Where can I download the ZBT-WG3526 OpenWRT source code?
  4. How can I configure he ZBT-WG3526 hardware specific hardware, e.g., GPIO

I've experienced on developing the Embedded Linux device, but it is the first time I using OpenWRT, so I need your help.

Best regards,

As a start: https://openwrt.org/docs/guide-developer/start