Initial sync is verification
During initial block download a node checks proof of work, chain linkage, transactions, scripts, issuance and other rules. The result includes the UTXO set needed to validate new inputs; a wallet separately tracks which outputs correspond to its keys.
Archive and pruning serve different needs
An archival node can serve old blocks to peers and applications. A pruned node validates first and then removes older block files under its storage target. It remains a fully validating node but may need to redownload data for a deep wallet rescan or reindex.
Resources involve trade-offs
Database cache, mempool size, connections, indexes and network modes trade RAM, disk, bandwidth and functionality. Defaults change between Bitcoin Core releases, so operators should use current documentation and configure for the service and threat model they need.
Reduce trust in the build too
Use official sources, verify signatures or hashes and, for advanced assurance, compare reproducible Guix builds. Back up wallet seeds and descriptors rather than the reproducible blockchain, protect RPC from untrusted networks and read release notes before upgrades.