The initial block download

What is happening

Your node first downloads all block headers from peers, which gives it the chain's shape and total work. Then it fetches blocks in parallel from several peers and verifies each one in order: proof of work, every transaction, every signature (for recent blocks), building the UTXO set as it goes. At the end it holds the complete current state of Bitcoin, computed entirely by itself.

Steps

1. 01Start the node and let it find peers. The dashboard or bitcoin-cli getblockchaininfo shows progress as a fraction of the chain verified.
2. 02Leave it running. Early blocks fly by; blocks from 2017 onward are full and verification slows. Expect several hours on a fast desktop, one to three days on a Raspberry Pi with an SSD.
3. 03If the node uses assumevalid (the default), signatures in blocks before a hard-coded recent block are not re-verified, which saves most of the time; everything else is still checked. You can disable it for a full re-verification.
4. 04Newer Core versions offer AssumeUTXO: load a recent UTXO snapshot, start using the node at once, and verify the full history in the background.
5. 05When progress reaches 100% and the tip matches a public explorer's height, you are synced. Your node will now stay within a block of the network.

Check your work

Run getblockchaininfo (or look at the dashboard). The block count should match the network height and verificationprogress should be essentially 1. Run gettxoutsetinfo to have your node report the total supply it computed. That number is 21 million being verified by your own machine.