I have found a few questions that relate to this (here and here) but I have been unable to interpret the answers in a way that I can understand how to do what I need.

I have 3 collections: Organisations, Users, and Projects. Every project belongs to one user, and every user belongs to one organisation. From the user's id, I need to return all the projects that belong to the organisation that the logged-in user belongs to.

Returning the projects from the collection that belong to the user is easy, with this query:

const projects = await Project.find({ user: req.user.id }).sort({ createdAt: -1 })

Each user has an organisation id as a foreign key, and I think I need to do something with $lookup and perhaps $unwind mongo commands, but unlike with SQL queries I really struggle to understand what's going on so I can construct queries correctly.

EDIT: Using this query

const orgProjects = User.aggregate(
    [
        {
            $match: { _id: req.user.id }
        },
        {
            $project: { _id: 0, org_id: 1 }
        },
        {
            $lookup: {
                from: "users",
                localField: "organisation",
                foreignField: Organisation._id,
                as: "users_of_org"
            }
        },
        {
            $lookup: {
                from: "projects",
                localField: "users_of_org._id",
                foreignField: "user",
                as: "projects"
            }
        },
        {
            $unset: ["organisation", "users_of_org"]
        },
        {
            $unwind: "$projects"
        },
        {
            $replaceWith: "$projects"
        }
    ])

Seems to almost work, returning the following:

Aggregate {
  _pipeline: [
    { '$match': [Object] },
    { '$project': [Object] },
    { '$lookup': [Object] },
    { '$lookup': [Object] },
    { '$unset': [Array] },
    { '$unwind': '$projects' },
    { '$replaceWith': '$projects' }
  ],
  _model: Model { User },
  options: {}
}