Three places your code runs

shared

shapes.lucent.ts

export type Point = { x: number; y: number };

export function nudge(p: Point): Point {
  p.x += 1;
  return p;
}

export class Counter {
  count = 0;
  increment(): number {
    return ++this.count;
  }
}

JS usage

App.tsx

import { Counter, nudge } from "./src/shapes.lucent";

const p = { x: 0, y: 0 };
const q = nudge(p);
p.x; // 0: p was copied in
q === p; // false: q is a new object

const counter = new Counter();
counter.increment();
counter.count; // 1: the same native object

What crosses the boundary

Value	Crosses as	So
number, string, boolean	a copy	the same value on the other side
arrays, object types, Record, Map, Set, Uint8Array, Date	a copy	changes on one side don't reach the other
class instances	a reference	the same object each time, with its identity
functions	a callback	Lucent can call JavaScript back
promises	a promise	await works in both directions
errors	an Error	with its message and code

Copying is what makes the boundary safe: no JavaScript object is shared with native code. It is also what costs time, so large data should cross rarely.

SDK objects, such as a CLLocationManager, stay in native code. JavaScript never sees them.