If you haven’t watched the time chain in action live, you can see a visualization of the time chain using mempool.space or time chain calendar. Both are great to explore different blocks and watch new ones move from the memory pool of all running bitcoin nodes to a confirmed block on the time chain. Each block is identified by its height or block number. A node is just a computer server running the software for bitcoin, lightning, and wallet software to interact with users.
Every block is created by a node that is also building blocks. They build blocks from transactions in their own memory pool. When a block builder proves it did the work of building the block by providing a hash of the block with the correct number of zeros at the beginning of the block header, it will broadcast the block to other nodes for them to validate. Once a block builder node receives a valid block, it will move to building the next block.
Given the speed of the internet and how quickly the information is spread, it can take a few minutes for that broadcast to get to everyone. And during that time, there are block builders that are still working on the same block. If one of them provides a hash of the block with the correct number of zeros at the beginning of the block header, it will broadcast the block to other nodes for them to validate. When this happens, some block builders will be building with different chain tips. To break the tie, the next block builder to provide a hash of the next block with the correct number of zeros at the beginning of the block header, will win the tie. Transactions in the block that lost, will become unconfirmed and will wait for a block builder to add them again.
To avoid any issues with spending bitcoin, waiting for 6 confirmations is a sufficient amount of depth in the time chain that the chance of a chain reorganization is nearly impossible. The amount of energy required to build a block becomes your protection from theft, it would take more money in energy than would be gained by trying to reorganize the chain.
