$lookup work in MongoDB but doenst work with mongoose
@Anthony Winzlet was correct, should be categories (in the pural) would have to leave categories (in the pural) and not category, but also, i had not defined the categoryId field as being an ObjectId in the Product Schema, so it was comparing string with ObjectId. In the tests in the terminal I had saved the server return, which returns the _id fields as strings. Now it's working.Thanks!
const ProductSchema = new Schema({ name: { type: String, required: true }, cod: String, storeKey: { type: String, required: true }, categoryId: { type: Schema.Types.ObjectId, ref: 'categories' }, description: { type: String, required: true }, price: { type: Number, required: true }, stockQuantity: { type: Number, required: true }, avatar: String, visible: Boolean}, { timestamps: true });