How does Node handle many requests if it's single-threaded?

Because most server work is I/O:

database
network
filesystem
API

Node doesn't have to block the main thread while waiting for these operations to finish.

For example:

const data = await db.find();

While it waits, the event loop can handle other requests.

That doesn't mean everything in Node runs on one physical thread, though. Node/libuv can use an extra thread pool.