There is a possible using a ["Field calculator"](https://docs.qgis.org/3.16/en/docs/user_manual/processing_algs/qgis/vectortable.html#field-calculator) inside the Graphical Modeler.

Let's assume there is a point layer `'test'` with some features in it, see the image below 

[![input][1]][1]

Thereafter I created a simple model for [reverse geocoding](https://en.wikipedia.org/wiki/Reverse_geocoding) as following:

[![model][2]][2]

*where* a "Field calculator" possess the following parameters

[![field_calc][3]][3]

In the 'Formula' tab apply this expression: 

    reverse_geocoding(y(transform($geometry,@project_crs,'EPSG:4326')), x(transform($geometry,@project_crs,'EPSG:4326')))

A custom `reverse_geocoding()` function defined as

<!-- language: python -->

    import geopy
    from geopy.geocoders import Nominatim
    from geopy.point import Point
    from qgis.core import *
    from qgis.gui import *
    
    locator = Nominatim(user_agent="MyReverseGeocoder", timeout=10)
    
    @qgsfunction(args='auto', group='Custom')
    def reverse_geocoding(lat, lon, feature, parent):
        try:
            location = locator.reverse(Point(lat, lon))
            return location.raw['display_name']
        except:
            pass

Here the Nominatim geocoder (free to choose) from the GeoPy geocoding Python library was used, for more details, please check [the documentation](https://geopy.readthedocs.io/en/latest/#nominatim). Also a point 

Mind that additionally installation of `geopy` library inside the QGIS is required, the *how-to process* described in [Installing 3rd party python libraries for QGIS on Windows](https://gis.stackexchange.com/questions/141320/installing-3rd-party-python-libraries-for-qgis-on-windows), [Installing Python packages in QGIS 3 (for Windows)](https://landscapearchaeology.org/2018/installing-python-packages-in-qgis-3-for-windows/), and [Install QGIS through OSGeo4W](https://www.giscourse.com/install-qgis-through-osgeo4w/) (also applicable for Python packages). 

And afterwards, I ran the model with [![run][4]][4] (<kbd>F5</kbd> as shortcut) as got the output

[![result][5]][5]


----------

Another solution that I may inspire with, described in [the @csk's answer](https://gis.stackexchange.com/a/300619/99589). So, you may try to find and download the reference point data with addresses for your area, and then simply using the ["Join attributes by nearest"](https://docs.qgis.org/3.16/en/docs/user_manual/processing_algs/qgis/vectorgeneral.html#join-attributes-by-nearest) geoalgorithm in the Graphical Modeler.


  [1]: https://i.sstatic.net/iJBsf.png
  [2]: https://i.sstatic.net/lJo3Y.png
  [3]: https://i.sstatic.net/5jdYZ.png
  [4]: https://i.sstatic.net/vHkKx.png
  [5]: https://i.sstatic.net/wIPoR.png