You can use directly the Keras function

https://keras.io/api/applications/densenet/#densenet121-function

or more generally look at here :

https://keras.io/api/applications/

Here an example for covid19-with W-Ray using Transfer Learning:

https://www.pyimagesearch.com/2020/03/16/detecting-covid-19-in-x-ray-images-with-keras-tensorflow-and-deep-learning/

Just replace VGG16 by DenseNet121

Have a look at https://www.pluralsight.com/guides/introduction-to-densenet-with-tensorflow

How image dataset import in colab for this?