Download Images to local storage

I have managed to read the links of images from firebase storage via getDownloadURL() and display it with . (using Angular)
But that has two disadvantages:
a) It has a delay of about 1-2 seconds until it is downloaded and then displayed
b) It is obviously not available without internet connection

Due to these disadvantages I would like to know how I can download the images onto the local storage of the phone?

Side question: How can I download several images in the background?