Let say we need to find if user is within polygon area.

Setting points of polygon

const p1 = [-122.45476, 37.77488]
const p2 = [-122.453, 37.76637]
const p3 = [-122.5104, 37.76397]
const p4 = [-122.51115, 37.77134]

db.areas.insertOne({
      name: 'Golden Gate Park',
      area: { type: 'Polygon', coordinates: [[p1, p2, p3, p4, p1]] }
    })

Set Index.

db.areas.createIndex({area: "2dsphere"})

Find if Point inside polygon area

db.areas
      .find({
        area: {
          $geoIntersects: { $geometry: { type: 'Point', coordinates: [-122.463792, 37.768638] } }
        }
      })
      .pretty();

db.areas
      .find({
        area: {
          $geoIntersects: { $geometry: { type: 'Point', coordinates:[-122.4436619, 37.774995] } }
        }
      })
      .pretty();

geoIntersects helps to find out area within polygon defined.