Nond

You need to use a name tag on the zombie to make it not despawn.

Did you give that item to the zombie or did the zombie spawn holding that item? Mobs only don't despawn if they picked up an item.

I tried giving it an item and it did not pick it up and it had gold boots on I was confused becouse I thought it would work. And the other one I tried picked it up but despawned so idk why that's happening

Just because a zombie already spawning in with an item doesn't mean it has the ability to pick items, those are two separate things on a Zombie.