updateOne returns a Query, which doesn't execute your update immediately. From the Mongoose guide for Queries it states:

A mongoose query can be executed in one of two ways. First, if you pass in a callback function, Mongoose will execute the query asynchronously and pass the results to the callback.

A query also has a .then() function, and thus can be used as a promise.

Meaning you can pass a callback function as in the documentation for executing Queries:

const Person = mongoose.model('Person', yourSchema);
// find each person with a last name matching 'Ghost', selecting the `name` and > `occupation` fields
Person.findOne({ 'name.last': 'Ghost' }, 'name occupation', function (err, person) {
 if (err) return handleError(err);
 // Prints "Space Ghost is a talk show host".
  console.log('%s %s is a %s.', person.name.first, person.name.last,
    person.occupation);
});

You can also use then or exec if you want to use Promises. From the Mongoose documentation for promises:

const query = Band.findOne({name: "Guns N' Roses"});
assert.ok(!(query instanceof Promise));

 // A query is not a fully-fledged promise, but it does have a `.then()`.
query.then(function(doc) {
 // use doc
});

// `.exec()` gives you a fully-fledged promise
const promise = Band.findOne({name: "Guns N' Roses"}).exec();
assert.ok(promise instanceof Promise);

promise.then(function (doc) {
  // use doc
});

If in an async method, you can also await the query:

await stock.updateOne({ticker: "TSLA"},
        { $push: { PastPrices:1}}
    );

Your query is correct, but in order for the query to return updated data, you need to pass additional config { new: true }. If you don't pass it, the query will update the data, but it will return old data.

stock.updateOne(
  { ticker: "TSLA" },
  { $push: { PastPrices:1 } },
  { new: true }
);