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The layered model, TCP versus UDP, congestion control, routing, and what happens when you load a URL.
3 questions in CS Fundamentals.
TCP guarantees an ordered byte stream and pays for it with head-of-line blocking and retransmissions that arrive too late to be useful. UDP is right when a late packet is worthless, when you need multicast, or when you intend to build your own reliability with different trade-offs.
A TTL is an upper bound on how long a resolver may cache a record, not a promise about when clients will notice a change. Lowering it only takes effect after one old TTL has elapsed, negative answers are cached under a different rule, and pooled connections keep using an address they resolved hours ago.
The browser checks its caches, resolves the hostname through browser, OS and resolver caches, opens a TCP connection, completes a TLS handshake, sends the request, then parses the streamed response. QUIC fuses transport and crypto setup into one round trip and stops one lost packet stalling every stream.