require(['uiRegistry'], function (r) {
    r.get("product_form.product_form.product-details.quantity_and_stock_status_qty.qty", function (el) {
        el.disable()
    })
})

require(['uiRegistry'], function (uiRegistry) {
    uiRegistry.get("product_form.product_form.product-details.quantity_and_stock_status_qty.qty", function (element) {
        element.disable()
    })
})

This answer is correct, however we're lacking information regarding how we know to use 'product_form.product_form.product-details.quanityt_and_stock_status_qty.qty' as the get() parameter.

.get is looking for its first parameter to be a component. So... how do you find the component, and is that language clear? No.

As an example, I wanted to add some custom javascript functionality to the customer information page within the Magento 2 admin. Specifically under, editing a user, 'Account Information' tab.

There are two ways to approach this from what I can tell...

Edit the datasource, to change values. Manipulate the field, such as visibility.

Edit the datasource:

Lets roll. Some dots are missing here, but we're on a page handled by the magento customer module, its an edit page... so we look here... /vendor/module-customer/view/adminhtml/layout/customer_index_edit.xml. Within here the content area is defined to contain 'customer_form'.

<referenceContainer name="content">
    <uiComponent name="customer_form"/>
</referenceContainer>

This means we're looking for a 'customer_form.xml' file, in this case it is found under /vendor/module-customer/view/base/ui_component/customer_form.xml. Within here we find the definition for a couple of the forms within the tabs and their fields. The datasource we're after is defined near the top.

<argument name="data" xsi:type="array">
    <item name="js_config" xsi:type="array">
        <item name="provider" xsi:type="string">customer_form.customer_form_data_source</item>
    </item>
    <item name="label" xsi:type="string" translate="true">Customer Information</item>
    <item name="reverseMetadataMerge" xsi:type="boolean">true</item>
</argument>

Note the 'provider' of 'customer_form.customer_form_data_source'. This is where all of our field data will exist within the .get call.

Our .get() would now become:

uiRegistry.get('customer_form.customer_form_data_source', function(element) {
    console.log(element.data);
});

Now, the question becomes... what can we do with this- and all you can do is adjust the data. You can turn fields true/false, manipulate values, so on. Useful.

Manipulate the field:

What if we want to hide the field and only show it under x condition... we can't do that with the datasource.

Now... this is going to be long and seem convoluted. Its because I'm teaching how to find your data... as I don't know how to find this any other way- and its going to be different per module. Teach a man to fish.

First I output my 'customer_form.customer_form_data_source' within our .get(). This will provide us a 'params' object key, within that is the definition of an 'activeArea' on the object, which is 'customer_form.areas.customer_edit_tab_view_content'.

Ok... so there are 'areas'. Areas, are the tabs on the left- each one is an area. Open up the '_elems' within your console, you'll notice 9 objects at the top. Open the second one and you'll find a label of 'Account Information'. This is the section I'm after. Then observe the 'name' value of 'customer_form.areas.customer'... ok lets try that as our get() param.

uiRegistry.get('customer_form.areas.customer', function(element) {
    console.log(element);
});

This is our fieldset wrapper, check its name value within the console under '_elems'->0->'name'. 'customer_form.areas.customer.customer'... we're drilling down. We're actually already there, check the 'name' value on the field you want to edit. You'll notice its name is 'customer[field_name]'. Making our path for get() is 'customer_form.areas.customer.customer.['field_name]'.

uiRegistry.get('customer_form.areas.customer.customer.[field_name]', function(element) {
    element.visible(false);
});

Examples: [form_name].[tab_areas].[area_name].[fieldset_name].[field_name] customer_form.areas.customer.customer.field_name