add ETH explanation

docs/tutorials/developer-journey/level-5/5.2-ICP-ETH-tutorial.md

@@ -378,6 +378,16 @@ Now, you can enter the Opensea URL that you previously copied to verify that you
 
 ![IC ETH 18](../_attachments/ic-eth-18.png)
 
+To verify this URL, the following is happening in the background:
+
+- The canister use HTTPS outcalls to send an [`eth_call`](https://docs.alchemy.com/reference/eth-call) RPC request to an Ethereum JSON-RPC API. 
+
+- This request involves encoding and decoding ABI, which is the Candid equivalent in the Ethereum ecosystem.
+
+- Then, the canister calls the ownerOf smart contract function for standard ERC-721 NFTs, and the balanceOf function for ERC-1155 multi-token contracts. This step checks that the user currently owns the given token. 
+
+- Then, the frontend reflects the result of whether the user owns the token or not. 
+
 :::info
 In this tutorial, you deployed this starter dapp locally. To deploy this dapp on the mainnet, run `dfx deploy --network ic`. 
 :::