And how are people buying/trading bitcoin now if they don't want to use third-party exchanges or custodial wallets?

I'm doing some research and writing about the history of bitcoin, and am curious if there are any OG bitcoiners out there who can tell me how it worked before bitcoin got big.

Hi, I'm new to crypto and still getting my head around how it works.

I've found charts on blockchain.com that show the network difficulty over time (https://www.blockchain.com/explorer/charts/difficulty).

Is there anywhere to find more specific information about difficulty-- eg. What is the current target value for candidate block hashes? Or, on average, roughly how many nonce values need to be tested (in the billions? quadrillions?) to find such a value?

Hi, I'm pretty new to crypto, and am trying to understand some of the technical concepts.

I think I get the concept of how miners run ASICs to find a PoW number that when SHA256 function is applied to the whole block with that number, it results in a working hash to make a valid block.

But they also need to validate each of the transactions within the block, right? And this requires checking the entire history of each of the included users' past transactions to verify that they have enough BTC to make a payment? For up to 2400 transactions per block, that seems very time consuming?

Am I misunderstanding how it works?

I tried to scan various FAQs and videos, but haven't found the answer to this question.

Thanks!