In the pre-ebook era we didn't have a lot of options for managing book highlights and excerpts. They generally lived on your shelf and if you didn't have that book with you, well, you were out of luck
The 2007 launch of the Kindle platform dramatically expanded the capabilities for highlights and excerpts...sort of. You didn't have to carry all those books around anymore but your thoughts were pretty much trapped in the Amazon ecosystem.
Not much has changed on this front over the past 10+ years but there are other tools that can unlock your book thoughts and notes. I'm talking about Evernote and how I use it to manage my book notes.
When I start reading a book I immediately create a new note in Evernote with the book's title. I'm reading more print books than ebooks these days, but the same approach I'm about to describe can be used for either. When I find a page or section I want to highlight or create a note about, I simply use the camera option in Evernote on my phone, take a picture of that page and stick it in the book's Evernote entry.
The result is a set of excerpts and notes that travel with me on all my devices. Better yet, I can share those notes with friends or colleagues. In fact, I'm using this solution right now to collaborate and share thoughts on a book I'm reading with one of my co-workers.
Evernote has optical character recognition (OCR) built-in and I often take pictures of hand-written meeting notes to save digitally. Oddly enough, Evernote is almost always able to translate my awful handwriting but it often has a hard time recognizing printed words on a book page photo. It works better on the Mac than my Android phone but it's still hit and miss. The downside is that your book page photos often aren't searchable within Evernote and I'm hoping they fix this soon.
Despite that issue, Evernote is a terrific tool for managing and sharing your book highlights, excerpts and notes.