To simply put, ETH and Smart Chain Network have the same address because they are both interoperable, and BEP-20, or Binance Smart Chain's token standard, is an extension of the ERC-20 protocol. That's why the BSC and ERC-20 receiving addresses in your Metamask wallet identical^[1]. You might want to read the article for a more technical explanation.

The only important thing you need to remember is to know whether or not a particular wallet/platform supports both network in terms of deposit and withdrawal to avoid a bit complicated solution for recovering your asset.

[1] https://academy.binance.com/en/articles/binance-smart-chain-vs-ethereum-what-s-the-difference