Bitcoin nodes still need a "seed" ip to connect to the first time. A few backup ips are hardcoded into the bitcoin core software in case you don't know any yourself. From then, the node will get and maintain a list of working peers.
The magic of the bitcoin protocol was to be trully decentralized. Meaning there is no central point of failure, not even a distributed common list.
But... How to achieve consensus in such situation would you ask ?
With the "proof of work". (for Bitcoin, but other cryptos are experiencing other things)
To push a bloc of transactions on-chain and be rewarded, a miner needs to first check the said transactions, and then be the first one to find a valid answer to an equation involving the block, solvable only by brute-forcing it but easy to check (more on that later). Once done, it spreads the block and the associated solution to his peers, that will stop trying to "solve", re-check the bloc you gave them and add it in order to start trying solving the next one. The difficulty of the problem is (slowly) auto-adjusting to have a bloc every 10 minutes, no matter the power of the network. The resulting difficulty level is a snapshot of the power of the network at work on the chain.
When 2 nodes are peering, they "handshake" each other, they basically tell one another "I'm at bloc number 456, my last block has this fingerprint".
If they disagree, the "longest chain" wins. But it's not really about size, longest means power speaking. Knowing the "difficulty level" of each block, you can do the math and know how much computational power was behind this version so far.
If it happens that the other node as a more followed truth, you start retrieving his blocks, one by one, from where you disagree, and you check them one by one, transaction by transaction, while checking that every proof of work is a valid one. At the end, you have another truth, but as you checked everything yourself and there is more PoW, you know it's a better truth. It's why it's better to wait a few blocs before saying that a transaction is unalterable, time for the consensus to take place, time for a "longest chain" to take over.
But this is really theoretical, as with the current hashrate and level of interconnection between nodes, a "longest chain fight" as infinitesimal chances to last more than 2 blocks.
The miner is allowed by the protocol to add one transaction to the block, his reward, from thin air. So if his block achieves consensus, he will be paid. Otherwise, it will have never happened.
This is how bitcoin achieve consensus.
You can't mine blocs with invalid transactions, as every node re-checks them, and as every node will check it too, you must have a valid proof of work. Then, if you want to delete a transaction from a bloc, it will change the equation to solve, so you need to find a new solution for this bloc, and if you want to win the longest chain war (as this bloc was already mined), you need to be the first one to close the next block.
So, to be able to change the chain, you need to be "in average" faster that the rest of the network to compute proofs, and raise your own version of the chain aside until you succeed in making it bigger, PoW speaking, than the main one. The famous 51% problem.
Now bonus: the bitcoin proof equation
Depending of the average block time, the protocol sets a difficulty level to keep it in the 10 minutes/bloc range.
Do you know SHA256 ? SHA is a hashing algorithm. It produce a 256bits "fingerprint" for a content. As close as you can study the output with different stuff, it will look like totally uncorrelated to it in any way, looks completely random. You change a single bit in the input, totally different output. But, and this is the attribute we are playing with, feed it with the same input, anytime anywhere, same output.
Each bloc has a header, a part including the hash (SHA) of the previous block and info about this one. The game is to find a number (called nonce), which added to the block header outputs a fingerprint starting by a certain amount of zeros. Yes, difficulty 14 means that a miner has to find a nonce that allows the output of a fingerprint starting by 14 zeros.
And as SHA is random-like, the only thing you can do is try a gazillion stuff, one by one, brute force.
Once you have a solution, it fits this specific bloc, as your nonce as to be fed through SHA with this specific header to outputs a valid answer, Both are linked for life. And as the header also includes the hash of the previous block, changing any block involves changing the upper ones.
So a proof is hard to compute with no shortcut other than brute-force, and checking just require to run SHA once by block. As long as the zeros are there, the power was. And here you go.
