I mostly rate books based on personal enjoyment but I try to be a little technical too. 1 and 5 stars are ratings I only give if I have some sort of bias, it's hard to be objective because I either found the book personally offensive or emotionally validating. I rate with my feelings anyway but in the middle of the scale I often manage to put some of them aside.

1 star - I actively hated it, and I wish I'd DNFed it or DNFed sooner (I rate books and write a review if I drop a book past the halfway point, because I think "this book was so bad I couldn't finish it" is still valuable information). (examples: Light From Uncommon Stars, The Midnight Library)

2 stars - I didn't like it but it wasn't necessarily bad. To me it was boring, forgettable. (examples: Bonds of Brass, She Who Became the Sun)

3 stars - I liked it but it was just okay. I may have a couple of major complaints or the book wasn't "for me" for whatever reason. (examples: Dune, They Both Die at the End)

4 stars - I liked it a lot and have no real complaints, might reread one day, but it lacked that little extra "something" that would make me love it and feel emotionally attached. (examples: The Blade Itself, Mort, 11.22.63)

5 stars - I loved it and really connected with it, it was the right book at the right time, I'll be talking about it for weeks and years to come and almost definitely reread it at least once. (examples: The Martian, Assassin's Apprentice, The Long Way to a Small Angry Planet, Nimona)

I usually write at least a small review to explain my feelings, so that if anyone is looking to ratings/reviews for guidance they can get a sense of whether or not they should take my rating seriously. While I agree that it sometimes feels unfair to rate a book low based on gut feeling alone (like you said, it's hardly the book's fault if it doesn't connect), it's also how I and many others read. A lot of books that for me were 1-3 star books are very highly rated on goodreads and some of my favourites have received bad reviews, so at least for me ratings are just not a good guide.

Highest of all my books is Way of Kings by Brandon Sanderson at 4.65 (others are higher but it's just people rating unreleased books 5 out of hype). I haven't read this yet but it's very high priority for me, can't wait!

Highest I've read is Harry Potter and the Deathly Hallows, at 4.62. It's nowhere near the best book I've read and IMO it's not even the best Harry Potter book but I don't really care about HP anymore so it's whatever.

Lowest overall and that I've read is Stjernetrekker by Harald Rosenlow Eeg at 3.00 (based on just 8 ratings). It's a norwegian book and a spiritual sequel to one of my absolute favourites as a teen, so I think very highly of it but I haven't read it in over ten years so I can't say if the low rating is deserved or not...

I honestly don't have a "usually" but I'm hoping to continue the trend from the last couple of years. I read three books in 2020 and maybe one or two? the year before that, and last year I read 46 (with a goal of 24). It's looking like I'll end this year around 40-42 (goal was 36), and I'll be very happy if I can break 40 next year too (tempted to make the goal 48)! I haven't read this much in over a decade and it's very exciting :)

Depends on how you define "worth it" but I've had a kindle for maybe a year now and I really like it. I still buy and read mostly physical books but my kindle has been great for a few things! I bring it when I travel, it's small and light so it doesn't take up half the space in my bag or weigh me down, and it's very handy when I'm sharing a room with someone who wants to sleep so I can't keep the light on. It also makes it easier for me to check out books I otherwise wouldn't have, since you can download samples or get good deals. It sucks way less to DNF a book when you didn't pay 30 dollars on a pretty hardcover copy! I also live in a small country so if I want to read something on release day that isn't right in the middle of the mainstream/insanely popular, the kindle version is my best option!

I've never been able to find the option to compare books in the app (android), I think it's only available in a browser (just like half the features I use sigh)

in a browser you can click a drop down menu on someone's profile and it'll show you all the books you've both added to your pages and give you a similarity percentage for books you've both read/rated. it's pretty neat :D