How does `bitcoind` generate the private key? My question is how the bitcoin core client generates a (master) private key(s)?

I know from other wallets, that they take the mnemonic and generate a HD wallet master key. With the master key, the other keys can be generated.

To my question. How does the bitcoin core software do it? I've never seen a mnemonic there.


Does it use a mnemonic?
I know about the backupwallet command to make a backup.
Is there nothing like a mnemonic I can use? If there isn't, do I have to constantly do backups?
Bitcoind does not support BIP39, which encodes a seed (128, 160, 192, 224, 256 bits) in a mnemonic phrase, from which the 512b HD root seed is later generated via PBKDF2 (Password-Based Key Derivation Function 2). This 512b root seed is used to derive the (extended) master keys.

For wallets without BIP39, the 512b HD root seed is directly supplied as entropy, without the intermediary step involving an encoded mnemonic phrase and PBKDF2. 

Backing up the wallet.dat file.
https://bitcoin.stackexchange.com/a/22722/90088 

Amended:
Bitcoin Core wallets will deterministically derive new extended key pairs and addresses as wallet usage progresses, which is reflected and updated in the wallet.dat file. However, when restoring a wallet from an older wallet file, the wallet will (re)derive all standard child key derivation paths, whilst (re)scanning the blockchain to determine past usage of these keys in transactions, thereby recreating the last used hd wallet state. The wallet can be fully restored, even if the wallet file backup is not the most recent one because of this rescan.