What happens after you press send?

Your wallet creates and signs a transaction. Peers check and relay it. A miner may include it in a proposed block; full nodes then check that block against the rules they run. A signature shows spending authority without exposing the private key.

Broadcast is not confirmation. A transaction can wait before it is included in a block. More blocks build on a confirmed transaction over time.

Source: How Bitcoin works

A transaction travels through BitcoinA wallet signs; peers check and relay; a miner includes the transaction in a proposed block; nodes independently verify the block. Lines illustrate a route, not a central operator. The moving packet illustrates this sequence, not live activity or elapsed time.Wallet signsPeer checksPeer relaysMiner: new blockNodes verify rulesNodeNode
A simplified route. Many peers participate, and each node verifies independently.

Do miners and nodes do the same job?

Miners spend energy competing to add blocks through proof of work. Full nodes check transactions and blocks. Finding proof of work does not turn an invalid transaction into one that a node will accept.

A blockchain is an ordered history of blocks. Its useful property is not that nobody can write data, but that participants can check whether the history follows their rules.

Source: Bitcoin developer guide

Where does the 21 million limit fit?

Bitcoin launched in January 2009. Its rules cap the supply at 21 million BTC. Those rules are checked by nodes; a supply limit does not guarantee demand or a price.

BTC is the asset. The network carries and checks transactions. Bitcoin Core is software used to participate in that network. These are connected ideas, with different jobs.

Source: Bitcoin FAQ

Does every payment need its own base-chain record?

The Lightning Network uses payment channels so each individual payment does not need a separate base-chain record. Opening and closing channels involves Bitcoin transactions. Liquidity, routing and custody still matter.

Source: Lightning Network paper

What does the diagram leave out?

Fees and confirmation times vary. Public transactions do not guarantee anonymity. A confirmed payment generally has no central operator who can undo it for you. Understand keys and recovery backups before handling funds.