You can import other Solidity files into a contract by using the "import" keyword followed by the relative path to the Solidity file that you want to import. Furthermore, in Remix you can directly import contracts from GitHub.
Code: https://solidity-by-example.org/import/
Open Zeppelin ERC 20 contract (sol 0.5)
https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v2.5.0/contracts/token/ERC20/ERC20.sol
Remix IDE: http://remix.ethereum.org
Solidity: https://solidity.readthedocs.io
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