Where and how to buy google play gift card with microsoft gift card or amazon gift card?

You cannot buy Google Play gift cards with Microsoft Rewards Points as they aren't available on Microsoft Rewards or Microsoft gift cards. If you happen to have a redeemed Amazon gift card tied to your Amazon account, you may be able to just search Google
Play gift cards on Amazon and purchase it with your available Amazon credit. Using the Redeem tab on Microsoft Rewards, going to shop, and then selecting the Amazon gift card (if you have enough points) may be able to get you a Google Play gift card you are
looking for on Amazon. Please understand that may work, it is not guaranteed to work.

As another user had pointed out, you could also try and redeem your rewards points for a Walmart or Target eCard (online gift card similar to Amazon) and buy a Google Play gift card through their websites or online or in store. Those are also not guaranteed
to work but may work.

Google Play gift cards aren't currently an available reward option but there are plenty others.

Hi

You can only spend your Microsoft Rewards points on the items within the Microsoft Rewards store:

https://account.microsoft.com/rewards/redeem/

Google Play gift cards are not available.

IRB is correct, you can no longer buy Amazon or Google Play cards (not sure you ever could get Google ones). However, there is still a Walmart card available. I could not find terms for Walmart cards that say you can't buy other giftcards with a Walmart
giftcard so maybe give that a shot!

You could also use your points to pay for gamepass!

No, you can't. Microsoft and Google both use different payment methods with regards gift cards so you won't be able to use a Google Play Gift Card to buy anything for Xbox.

The £10 must be either on a card that says "Xbox Gift Card £10" or you must purchase an Xbox digital gift card code on Amazon for £10 that you redeem here:
https://redeem.microsoft.com/enter

You can't redeem Amazon gift cards or currency in Amazon accounts for currency in your Microsoft/Xbox account - you must buy an Xbox Gift Card (or code) with that money on it.