So I can randomly select 12 words from the 2048 and generate a bitcoin wallet? Thats pretty neat
No, you can't.
So far every 12 words I've chosen have not opened a wallet
The final word in every seed phrase contains a checksum. If the checksum is not correct then usually the software you are using will not accept the seed phrase. By randomly picking words, you only have a 1 in 16 chance of selecting a word with the correct checksum for a 12 word seed phrase. This falls to 1 in 256 for a 24 word seed phrase.
But more importantly, selecting words yourself is a terrible way to generate a wallet, will massively decrease your security, and could easily lead to loss of funds. Don't do it.
How come the fees for legacy addresses are so much high then addresses that start with a 3?
Some address which start with a 3 are nested segwit addresses. Segwit moves part of the transaction data (the witness) to a separate section and counts it differently when calculating the weight of the transaction. Since these transactions now have a lower weight, they will pay a lower fee. Addresses which start with bc1 are native segwit addresses and will reduce the fee even further.