Swaps on the Uniswap Protocol can start and end with ETH. Depending on the version of the protocol the swap uses, ETH may need to be wrapped into WETH during the swap. On Uniswap v4, it is not necessary to use WETH to swap ETH.
This is because in order for tokens to be swappable on the Uniswap Protocol, they must be an ERC-20 token. However, ETH was built before ERC-20 standard. This is why we use WETH. WETH is an ERC-20 token and can be used to swap with other ERC-20 tokens on the Uniswap Protocol.
Thus, ETH can be swapped on the Uniswap Protocol but it is wrapped into WETH during the swap.
Note: If you attempt to swap ETH using UniswapX, you will be prompted to wrap your ETH into WETH before you are able to sign the swap.
This is because all native network tokens (ETH, MATIC, etc.) must be wrapped (WETH, WMATIC, etc.) before they can be swapped on UniswapX.