The main interest here is for recommended advice and or resources, such as hard copy or virtual print, audio/video/web sites, software, workshops or the like , that are intended to allow a lay person a self-paced study of the subject so that they can develop a practical understanding IE. not a doctorate or professor / guru level but nonetheless understanding the basic fundamentals that then compliment their self-paced enthusiasm/learning/study of playing piano.
I remember first beginning piano lessons 50 years ago, enough to remember that 1) i did not ask many questions, i simply followed the teacher’s guidance and 2) i do not remember her bringing up the topic - music theory - but now that i’m on the other end of 50 years, i think & feel obligated to ask questions….. i really am at the opposite end of asking questions spectrum compared to where i was as a 12 year old. At the same time, i realize there is so much varied history to music theory that some seemingly simple (burning) questions may never be answered with short, simple explanations, so i expect that, in the interest if time, the explanations for some fundamentals may need to be taken for granted when understanding remains elusive.
For example, intervals: i understand - to my satisfaction - those that begin with the second and follow forwards from there all the way up to an eighth or octave and beyond, mainly because i can both see and hear the difference / space / gap / distance theory calls “interval” …. but after many attempts to stand under this most fundamental concept. i still fail to understand how pressing the same key twice , yielding a gap of zero distance/pitch, constitutes the first interval , be it on an actual AP where one key may be striking 1, 2 or 3 strings or on a DP attempting to mimic same 🙁 🤔