How are 21 words seeds generated Apologies for the noobie question.
I was wondering how a range of wallet addresses (eg. BTC address 33ELbMhwHw5AaKHU4u3MzVSFvvK7aa7HZL) is algorithmically converted into seeds (eg. 12 words). Any help?

I tried to google around but I only find articles related to ledgers, wallet recovery and so on.

Thanks
The base system is described in BIP32 and BIP39.

Essentially, the conversion goes the other way around. We don't convert a list of addresses to a seed. A seed is converted to a list of addresses.

BIP32 described how hierarchical deterministic (HD) wallets work.

BIP39 describers how these can be encoded into mnemonic phrases, or seed words.

There's an excellent open source tool by Ian Coleman - BIP39 that lets you play around with the various derivation and entropy options. You should, however, be wary of entering any seeds containing funds or using it to generate secure seeds, as always.