Since these notes are for myself only, I can be as stupid as I wish in these notes, even making outrageous blunders in my commentary. I often catch the blunders on rereading and make new commentary on them. The presence of such errors helps me see where others may go astray when I am writing up my own work. I write somewhat better as a result. I am sure you have noticed that much of what we mathematicians write consists of lots of details used to make a few straightforward ideas intelligible and believable to the reader.
Many proofs consist of one or two ideas and a lot of supporting detail.
Having noticed one or more ideas in a paper, if any of these ideas seems particularly interesting, I will write it into my summary. It helps me remember the idea and maybe use it in my own work. Only a genius could routinely find the key ideas in a quick reading of a difficult paper. Of course you are right about the difficulty of catching the ideas. Fundamental ideas may or may not be highlighted by the author of a paper.