Work through these books, then you start looking at any random app or system and can start from there:

- System Design Interview – An insider's guide: https://smile.amazon.com/gp/product/B08CMF2CQF

- System Design Interview Vol 2: https://smile.amazon.com/System-Design-Interview-Insiders-Gu...

- Designing Data-Intensive Applications: https://smile.amazon.com/gp/product/1449373321

- System Design Primer: https://github.com/donnemartin/system-design-primer

Especially: https://github.com/donnemartin/system-design-primer#how-to-a...

- The Architecture of Open Source Applications: http://aosabook.org/