sibling comment with links to docs is the more accurate, but to summarize, it's some of both:

* all connections are always e2ee (even when traffic flows through a relay)

* relays are both for connection negotiation, and as a fallback when a direct connection isn't possible

* initial packet is always sent through the relay to keep a fast time-to-first-byte, while a direct connection is negotiated in parallel. typical connections send a few hundred bytes over the relay & the rest of the connection lifetime is direct