1.) Understand the SOLID principles and WHY they matter: -Single Responsibility Principle -Open-Closed Principle -Liskov Substitution Principle -Interface Segregation Principle -Dependency Inversion Principle
2.) Understand the 4 principles of Object Oriented Design and WHY they matter: -Abstraction -Encapsulation -Inheritance -Polymorphism
3.) Understand the 3 types of design patterns: Creational, Structural, and Behavioral. Understand the problem each solves, their respective uses, and which principles they either implement or achieve.
4.) If you write web services (i.e. SOAP/REST), understand the 12 principles of Service Oriented Architecture (i.e. no God classes). Microservices have their own patterns you can explore like Strangler, Saga, orchestration, choreography, etc.
5.) Read "Refactoring" by Martin Fowler (1999) - great book that describes how to approach refactoring application logic safely with improved results, like how to approach a refactoring huge (hard to test and maintain) block of if/else's into a Strategy Pattern, as an example.
6.) Read books written by "Uncle Bob", Robert C Martin: a) Clean Code: A Handbook of Agile Software Craftsmanship (2009) b) Clean Coder: A Code Of Conduct For Professional Programmers (2011) c) Clean Architecture: A Craftsman's Guide to Software Structure and Design. (2017)
7.) READ "Design Patterns" by the "Gang of Four" (GoF) as they're known, Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1995). THIS IS MINIMUM CRITICAL KNOWLEGE.
8.) Read "Domain Driven Design" by Eric Evans (2004).
I hope you don't view the reading this as punting the answer, but time spent reading any of those should help you in your quest to get better at software architecture.