Don't reinvent the wheel. Depending on the role you plan to take on, building everything from scratch can easily become a massive waste of time. There's plenty of resources online and it's a bit overwhelming . Below are some hand picked links to get you started.
Mocap
Mixamo.com - mocap & Characters
charactergenerator.autodesk.com - custom characters & crowd
Plates
Pexels.com - plates, watch out most are in slow mo
https://mango.blender.org - 4k exr plates
3d / 2D Assets
polyhaven.com - hdri, textures, models
quixel.com/megascans - textures, models, free offline library with brick and mortar schools
sketchfab.com - virtual museums of Malopolska
cgtrader.com - low grade models and some usd assets
Examples
cgwiki.com - houdini tips and tricks
https://hdbp.io - houdini tricks on a browser.
RBD
http://nicholas-taylor.com/assorted-bullet - assorted-bullet sims and example hip file
Data
https://physicallybased.info - camera, color, material properties.
Karma
https://www.sidefx.com/tutorials/karma-crash-course - 3 part series
Below is a small collection of my karmalicious renders from example files showcasing procedural stuff and making use of online resources. Each setup is done in less than 6 hours and rendered in approx.2 hours using Karma XPU.