user106548: So Bitcoin uses an HD wallet and thanks to that it can store keys in a hierarchy. But how exactly and in what format are they stored? Is each private and public key stored as a separate file? Are they stored in textual or binary format? Does Bitcoin use a database to store them?
The only resource I found is the source code of Bitcoin Core but are there any books or sites where I can find explanations about this?

user26873: No they are not stored, the child keys are calculated from the master when needed as they are deterministic, not random. 
Here are some references:    
https://www.oreilly.com/library/view/mastering-bitcoin/9781491902639/ch04.html#_deterministic_seeded_wallets
https://learnmeabitcoin.com/guide/hd-wallets

106548: <p>So Bitcoin uses an HD wallet and thanks to that it can store keys in a hierarchy. But how exactly and in what format are they stored? Is each private and public key stored as a separate file? Are they stored in textual or binary format? Does Bitcoin use a database to store them?</p>

<p>The only resource I found is the source code of Bitcoin Core but are there any books or sites where I can find explanations about this?</p>
 26873: <p>No they are not stored, the child keys are calculated from the master when needed as they are deterministic, not random. </p>

<p>Here are some references:    </p>

<p><a href="https://www.oreilly.com/library/view/mastering-bitcoin/9781491902639/ch04.html#_deterministic_seeded_wallets" rel="nofollow noreferrer">https://www.oreilly.com/library/view/mastering-bitcoin/9781491902639/ch04.html#_deterministic_seeded_wallets</a></p>

<p><a href="https://learnmeabitcoin.com/guide/hd-wallets" rel="nofollow noreferrer">https://learnmeabitcoin.com/guide/hd-wallets</a></p>
