The messages are being sent to you as iMessage, which is why you receive them on your iPad and not your Android phone. If you want text\iMessage to continue being sent to your iPad there is nothing to change. If you want the messages only coming to your Android phone you need to log out of Messages on your iPad. On the iPad in Settings -> Messages turn off iMessage using your Apple ID (email address).
Make sure your phone number is deregistered
https://selfsolve.apple.com/deregister-imessage/
For group texting to work, make sure RCS Messaging is enabled on your Android phone, and use a message app that supports RCS messaging (Google Messages). Make sure whoever texts you using an iPhone has RCS messaging enabled.
Learn why some of your message bubbles are blue or green.
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