How PEX protocol (Magnetic links) finds it first I

I'm trying to understand how can a magnetic link work, as I've read they use DHT and PEX to get the peers, but if I'm a new node in the network how can I find peers with only the hash of the file?! Doesn't it always require a link to a known host?

Thanks

The bittorrent DHT can be bootstrapped in many ways. It just needs the IP and Port of any other reachable DHT node out there.

Current clients generally use several of the following strategies:

bootstrap from a cache of long-lived nodes from a previous session
use a DNS A/AAAA record mapping to a known node (e.g. router.bittorrent.com or dht.transmissionbt.com) with a known port
use a node embedded in a .torrent file
retrieve the DHT port from a bittorrent client over a bittorrent connection established through other means, e.g. a conventional tracker.
- If a peer is embedded in a magnet link one can also piggyback a DHT bootstrap on that through the port message
multicast neighbor discovery via LSD
cross-chatter from the IPv4 to the IPv6 DHTs and vice versa (if needed)

Other ways such as user-configurable bootstrap lists, DNS SRV records round-robin mapping to live nodes or - should everything else fail - adding the IP of your friend(s) manually work.

Once a node has joined the network the first strategy mentioned above will kick in and it is unlikely that it will have to bootstrap again.

So while most implementations rely on a single/few points of entry into the network for convenience, the protocol itself is flexible enough to decentralize the points of entry too.

Just for emphasis: Any node in the DHT can be used to join the network. Dedicated bootstrap nodes are an implementation detail, not part of the protocol, and could be replaced by other discovery mechanisms if necessary.