An example is there for users to learn from and adapt to their needs.
I thought you could easily find out if there exists something similar to WiFiClient for Particle, or get help for this in this forum. I do not decline to help you, but you could start with searching on Particle.
I have started a topic in this forum to get help for me to help:
There are numerous httpclients in the library and now httpsclients so strike requirement 1.
I have done a HTTP download to an SD card of a .bmp file but never got around to getting it to work for HTTPS.
It should be possible to combine the GxEPD2_WiFi_Example (where all the hard work in processing the .bmp format to write to the EPD has been done by Jean-Marc) with the httpsclient to get something that can would on the Particle platform.