Understanding BIP84

I try to understand how the seed phrases root, master keys and BIP84 is working.Found this site: https://bitcoiner.guide/seed/

BIP39 is clear so far: I can create 12 (or more) words + passphrase which prdouces a hex seed.But there is also a BIP32 Root key, not sure how that is created but somehow it's derived from the seed I guess.

In the next section Derived Addresses I use BIP84 and the inputs from https://github.com/bitcoin/bips/blob/master/bip-0084.mediawiki. So I'm using 11x abandon and 1x about without any passphrase. The wiki shows these keys:

• rootpriv
• rootpub
• xpriv (Account Extended Private Key)
• xpub (Account Extended Public Key)

and of course the other derivated addresses, private and public keys using the path levels.

On the bitcoiner.guide the xpriv and xpub are called:

• Account Extended Private Key
• Account Extended Public Key

the rootpriv and rootpub values are not shown anywhere, but in addition it has these:

• BIP32 Extended Private Key
• BIP32 Extended Public Key

This is really confusing a bit, anyhow my questions are:

1. I don't need to backup anything except the BIP39 Mnemonic and the optinal BIP39 passphrase, right? Except I'm using a 2/3 multi sig wallet. But my focus is BIP39 and BIP84
2. How are the derived addresses with this kind of path level m/84'/0'/0'/0/0 are used?Is the software (like Electrum, etc.) using automoaitcallly a new address for each transaction or do I need to increment the index in the path level manually?
3. If I create a BIP32 wallet in sparrow, it shows me both: a xpub and zpub key. That key is not shown, neither on the mediawiki page nor in the bitcoiner.gudie
   Sparrow shows me this descriptor: wpkh(BIP392) - what is that?