I get some of your explanations up but my question is, the 24 words long is that the highest that can a seed phrase is to have. Because I read that seed phrases although are produce by the wallets but they also can be customized by the user (although senior members warn against it because of lack of randomness). Can one not customize a seed with longer words say 48 just to have a much more bigger bits of entropy I.e larger than the 256 for 24 words

The seed phrases cannot be more than 24 however, trying to customize even with more than 24 words and up to 48 as you suggested or more won't lead to security enhancement of which is primary to the concept of seed phrases because you cannot be more random and secured than the traditional BIP39 standard seed phrases, even as it comprises of a list of 2048 words.