Your question is not clear, as your comparison is probably not accurate.
I use a simple explanation here. Mainnet is the main network where everything happens in real time and is the actual blockchain network. This is different from Testnet where it is the same blockchain protocol, everything works exactly the same,,, but developers only use it to try out new technology, hence Testnet.
For example, Bitcoin Mainnet is where everything we transact takes place. Bitcoin Testnet is exactly the same, but where we can try out new tech like Lightning, without risking actual Bitcoin. Also there are almost no miners securing Testnet.
Public blockchain is compared to Private blockchain. Public is like Bitcoin, open source and available on github where we can all see the code. Private is owned by someone, and code source is closed (we cannot publicly view).
Thank you for the explanation. Basically, why i'm asking was there are some altcoins from ERC20 Network went to their own private/public blockchain as we called their own "mainnet" such as TRX for example. I was wondering what is their purpose to do that, is public/private blockchain has more advantage for some reason other than using ERC20 Network?
Ah, that is easy to explain for me right now. First of all, the Ethereum network is getting more and more congested. So users especially newbies are finding a lot of transactions never going through, because they do not know how to set high fees. Those who know, feel the fees are just too high. Moving to another faster, less crowded blockchain means faster and cheaper fees.
Moving to own mainnet also means they have more control. Their previous tokens were just protocols on Ethereum which they have to follow. But if you make your own blockchain of course you can customize it to your own needs.
