Are my NFTs on Universe visible on other platforms?

In short— yes.

NFTs are minted on the Ethereum blockchain which means you can view them on any platform that you desire. For instance, an NFT minted on OpenSea will be visible on Universe and vice versa. If listed and sold on one platform, you will not be able to sell that NFT again on another platform.

Keep in mind, some platforms may not show your NFT for varying reasons. OpenSea requires collections to be verified, similar with Rarible. You may need to directly search up your personal address or collection address on other platforms.

You will always have access to your NFTs on other platforms if you connect the same wallet used on Universe XYZ. But, there may be conditions you need to fulfill first so that your NFTs display across various marketplaces in public view.