We have over 2048 words on the BIP39 word list,
We have multiple word lists each in a different language but in each word list there is exactly 2048 words (not over it).
this means that a string of 12 words would have 128bits of security.
To be clear this is a combination of the number of words and the length of the word list but generally speaking the security of the mnemonic comes from the size of the entropy. A 128 bit entropy providing 128 bits of entropy could be encoded using a bigger list to have smaller number of words or a smaller list and have larger number of words but the security remains the same.
If an intruder wishes to attack a wallet it is expected of such to carryout 2128 operations to guess a 12 word phrase.
It is 2
128 not 2128 and it is not the number of operations, it is the number of possible values for a 128 bit entropy.
When copying text from elsewhere you should take note of the formatting!
It might seem to be a small number, but it is more complex than you think. This seed can also be extended to improve security.
Extension most of the times does not provide additional security because for one users don't use a truly random input as their extension word and also the algorithm used to derive the keys is not strong enough.
Seed phrase extension
Seed extension is the process of adding more words to the existing seed phrase to strengthen the security of a crypto wallet.
Not exactly correct. I alraedy explained why it is not providing security, also it is not "adding more words" it is adding an arbitrary string which can be words or literary anything else like a bunch of emojis or a random password like "jz7P*s&{$Ys5+GoJoA".