Where is seed in Bitcoin Core in comparison to Bread Wallet Im new to Bitcoin. I know the main software of bitcoin is Bitcoin Core. Recently i started using an iOS wallet called Bread. Its very simple! It gives me a 12 word key and says your master private key can extract from this and no need to backup your wallet as a file.

My question is: i readed that Bitcoin Core in the new versions is an HD Wallet. But it never gives me a seed like bread. Im confused.

I have an assumption but i dont know its right or not: 
My master private key stored in a file unlike bread. I have to encrypt my walled from bitcoin core settings menu and the from file menu choose Backup Wallet and save it. In fact, in the future, i have to import that file and enter encryption passphrase that i created in the encryption step and the software fetches my private key from that file and load my entire wallet from blockchain. In bread i enter that 12 word and bread creates my private key and so on... in bitcoin core i need backup just one time. In Bread i have to write down my seed and in Bitcon Core i have to backup both passphrase and wallet file.
is this true? bitcoin works like this?

As i know i cant import wallet i created in Bread to bitcoin core. Each software has its own seed to private key mechanism. Is this true? I want to use bread and Bitcoin Core both in my pc and mobile.
In Bread i have to write down my seed and in Bitcon Core i have to backup both passphrase and wallet file. is this true?


Yes.


  Each software has its own seed to private key mechanism. Is this true? 


No. What you call the "seed" is actually called the mnemonic. Most wallets that give you a mnemonic use BIP 39. BIP 39 specifies how the mnemonic should be created and how to convert the mnemonic to an actual seed value.  Most HD wallets then use BIP 32 to convert that seed into your private keys

Bitcoin Core implements BIP 32, but not BIP 39.