Manuals. Compilers came with comprehensive manuals. When you paid for things like Turbo C++ or Paradox or MS Cobol you got a well edited, indexed and quality printed book that covered the entire language and tools comprehensively. From there you wade into existing code bases, learn by maintaining the work of others, and eventually you originate new work. Along the way you dog-eared the manuals.

The Internet has obviated the need for comprehensive books for the bulk of contemporary programming. I suppose there are obscure enough and/or proprietary platforms that still follow the "comprehensive documentation" approach, but it seems it's possible for widely used tooling to be poorly documented and rely on online forums to fill in the gaps with crowd sourced answers and examples. This makes "mind share" crucial to the usability of a language/platform.

I am sometimes frustrated with the contemporary model, but the old way wasn't a panacea either; just broken differently. If you want a good taste of "what it was like" find the manuals for Turbo C++ 3.0 (1991.) You'll learn things about C/C++ that you've never seen written down anywhere.