Technology like Beowulf clusters and Kubernetes has made it easy to build clusters using off-the-shelf components. In this video, Jeff introduces the concept of Raspberry Pi clustering, and cluster computing in general.
YOUTUBE kgVz4-SEhbE Introduction to Pi Clustering - Jeff Geerling
More info about the Pi Clustering series and links relating to topics discussed in the video 
Also see
1/4 the cost!
Turing Pi Clustering video
Raspberry Pi as a service is here (pricing for the Pi 4 server starts at £7.25 per month). page
Raspberry Pi Dramble site is being served from a Drupal site, running on a Kubernetes cluster of Raspberry Pi computers, in Jeff Geerling's basement.