3. Why aren't developers tweaking the Bitcoin blockchain code to accommodate millions of transaction like Solana and BNB?
I don't know Solana but BNB is a centralized shitcoin made up by a rich guy. A centralized database can't be compared to a decentralized blockchain.
There are decentralized chains that have higher TPS than btc. They can make blocks bigger and/or more frequent (and reduce reward per block) to increase tps but looks like no one care that it's really hard to use L1 BTC as payment.