I think writing by heart is like playing by ear. You aren't aware of technique while you do it, you just do it. Right now in my writing journey, I don't think much about the mechanics of writing. I just write.
I remember reading books at a young age and more than the story jumped out at me. Hard to explain except to say the words used, the sentence structure, and all the mechanics of writing stood out. Victoria Holt and her historical fiction novels taught me a tremendous amount about foreshadowing and suspense and tight writing. L.M. Montgomery taught me how to write with feeling and color. Beware of purple prose!
Many writers are wired to write from a young age. They go on to write for so many years and develop their craft to such an extent that the spirit takes over. They now write by heart and they get it right. You'll never have good writing without pouring your heart into your words.
Lately, as I edit/rewrite/polish this second book, I pray for an editor's eye. And surprising things happen. I see things that really don't work or need to be tossed. I am fond of overwriting (editors are not)! I love to take rabbit trails just for the joy of writing. Sticking with the story and making every word/sentence/paragraph contribute to the plot is not nearly as much fun. But I have to do it. And it makes for a better story.
Book 1 is now with the copyeditors (is that one word?). I am curious to see what they do exactly. This is all such an adventure. I wonder if writers who have published for years lose their excitement over the publishing process. Can anything be as thrilling as holding your first book in your hands? I hope that I never lose the gratitude I feel right now.
The steps of a man are established by the Lord, and He delights in His way.