Not just professional equipment, pre-90s every electric/electronic hardware itself larger than a book had at least a high level block diagram on a sticker on the back or on an inside wall, and also more detailed diagrams on manuals. Complexities then went overboard and those diagrams became rarer.

I remember those diagrams also didn't include mechanical drawings, so the last part of your comment is right too. You aren't going to be making magnetic read/write heads or even DC motors from information taken from those diagrams, merely the topology of control circuits - which is an opposite view from how we'd see it today.