210_000 in ” uint256 internal constant FUNDS_SEND_GAS_LIMIT = 210_000;”

SafeMath is a solidity math library especially designed to support safe math operations: safe means that it prevents overflow when working with uint. You can find it in zeppelin-solidity SafeMath.

From the official documentation:

The directive using A for B; can be used to attach library functions (from the library A) to any type (B). These functions will receive the object they are called on as their first parameter (like the self variable in Python).

The effect of using A for *; is that the functions from the library A are attached to any type.

Hope it helps~

That is not a transfer operations but an event notification.

This is done that way so block explorer and wallets that only interpret the 'Transfer' event can show the operation to the user. There were some complaint from users that the transfer failed because it didn't show up in their wallet (or block explorer).

Only the 'Transfer' event is part of the ERC20 standard, and the events 'Mint' and 'Burn' are not. It is a workaround around that limitation to generate a tranfer from 0x0 to simulate a 'Mint' event and, a transfer to 0x0 to simulate a 'Burn' event.