I didn't mean to redeem the gift card to my account, can I transfer it to my fiance?

Once it's been redeemed, there's no way to transfer it. You could always buy something as a gift so they'll get a code for it.

Would like transfer fund into my grandson Microsoft account, it not family account.

Hello, you are unable to transfer the funds directly to your grandson. You may however purchase a digital gift card that will provide a code that can be redeemed on the Microsoft Store, directly from your grandson's Xbox. I have provided two links. One in
which you can purchase the digital gift card, and another that shows how to redeem codes.

Digital Gift Card

https://www.microsoft.com/en-us/p/microsoft-gift-card-digital-code/cfq7ttc0k61w

Redeem Codes

https://support-origin.xbox.com/es-EC/billing/prepaid-codes/redeem-prepaid-codes

I accidentally redeemed xbox gift card to my microsoft account instead of my son’s. I dont have an xbox account but need to transfer the funds to my son’s account. I read that transfer isn’t possible. Is there any way?

No there isnt any way. Nothing is transferable between accounts. What you can do however is use the balance on your account to gift a game of your sons choosing to their own account by selecting "buy as gift" when purchasing whatever game they want.

Hello, I've already bought a gift card for my nephew but unfortunately I didn't know that it should be just by his account to be able to use of the 25 digit number code for gaming. Is there any way that I transfer this gift card from my Microsoft account
to his account?

Thanks,

Leila

No unfortunately not. Nothing is transferable between accounts.

There's no way to transfer items/content redeemed on another account. If you redeemed a currency card on the account, you can buy the item as a gift for your other account.