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To answer your question of whether or not an account can hold tokens from several blockchains, the answer would technically be no, however an Ethereum (ETH) account can hold a variety of tokens that can exist on other networks.
MATIC is a good example of this situation – you can have Polygon-native MATIC, as well as the ERC-20 (Ethereum compatible) version of MATIC as well. You can hold your ERC-20 MATIC in your ETH account, however this is because Polygon in and of itself is an EVM-compatible (Ethereum Virtual Machine) sidechain.
With respect to ADA, you cannot hold ADA in an ETH account (it will likely throw you an error saying that the address is incorrect, if you do try to send ADA to an Ethereum account).
Ideally, you want to have respective accounts for each of your different cryptocurrencies, however you should be able to store ERC-20 tokens all in a single Ethereum account (meaning you can hold SHIB, MATIC, AAVE, LINK, and many more tokens in an ETH account).