Is there a free API that gives geojson data for borders of different cities in the world? [closed]

I am trying to create a maps application that creates a color coded polygon over the city based on air pollution levels. I could not find any API provider that provides the data clearly. This is what I need the response to look like or include:

"geometry": {
    "type": "Polygon",
    "coordinates": [
      [
        [ 44.55894, 54.30577 ],
        [ 44.55198, 54.30724 ],
        [ 44.5507,  54.31366 ],
        [ 44.54092, 54.32204 ],
        [ 44.54182, 54.32476 ],
        [ 44.54499, 54.32563 ],
        [ 44.56358, 54.32214 ],
        [ 44.5689,  54.31886 ],
        [ 44.55894, 54.30577 ]

I found API by https://www.geoapify.com/boundaries-api but this one only gives the borders of places that make up the city and not the entire city. I also found this GitHub link https://github.com/drei01/geojson-world-cities but I have hard time understanding how to use this to get the data I need.

Any advice or help would be appreciated!