Tutorial request = music theory

i love al the how to make / sound like tutorials and find this way of learning by far the best method … i am starting to learn more and more about using a synth to sculpt sounds, using effects and plug ins to tweak … but having no real music theory knowledge (other than the music theory courses on here and the odd you tube channel) br
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i wouldnt mind knowing for what reason the notes in the melodies of the tutorials you provide are used … if that makes sense, its hard to say in words what i mean … in other words are you moving from dominant notes to leading tones etc etc br
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or is it just a case of hitting any note (within the key of track) and seeing what works … or does all music / melody writing need to follow a certain structure br
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hope this is clear what i mean :w00t: br
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i tend to find myself watching tutorials but always wondering why certain notes are played, do you always start a chord prgression with the key of the track, is the first down beat of a bassline always the root note of the key etc