Hi there , I’m Jeremy.
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According to 101blockchains, a mainnet is a blockchain that performs the functionality of transferring a digital currency from a sender to a recipient. Mainnet is simply the main network, whereby actual transactions take place on a distributed ledger. In other word, a mainnet is the main blockchain network for a project to run and seemed to be the final product of it.

In contrast with mainnet is testnet. A testnet is just a prototype blockchain where programmers and developers test and find faults or bugs...

Mainnets are usually followed by mainnet swap. As mentioned above, mainnet is the official (also native) blockchain of a project. After a mainnet is launched, that whole project must migrate from the old blockchain to the native one. That process calls mainnet swap.

Thank you.