I think you mean toppers like these?
https://www.highlinewigs.com/s.....op+Toppers
Basically mini-wigs that just cover the top of your scalp (and they come in all different sizes, depending on the coverage you want/need). Different from extensions, which I agree aren't helpful if you don't have the thickness up top to cover the clips. But toppers are exactly made for thinning crowns (or thinning all over), and a good topper can look completely natural.
I've played with them a little, though my hair hasn't gotten quite thin enough yet to wear them regularly (it's been somewhat thin with no obvious cause since at least my late twenties, sometimes a little better and sometimes a little worse, though I don't think most people but me --and my dad -- notice). I feel better knowing they're an option. Highline Wigs has a great Instagram account with lots of gorgeous women wearing toppers and wigs (sometimes showing them with their natural hair and with the topper so you can see the difference), here: https://www.instagram.com/highlinewigs/
My primary care doctor asked at my recent physical if my hair was getting thin, and asked if I'd like to try medication for it (it was, and I did). I've been on oral minoxidil for about two months now, not enough time to tell what the results will be because hair is sloooooooow. But I've noticed a difference in all my other hair, which is a mixed blessing (so far worth the very slight extra hassle, and I have fuller brows and more than 3 undereye lashes for the first time in my life!). I *think* I have a lot more baby hairs coming in around my hairline, so I'm hopeful. Minoxidil isn't right for everyone given potential side effects, it doesn't work for everyone, and you have to stay on it to keep the effects, but if you're interested in exploring that or other options, find an understanding doctor and ask (lots of dermatology practices are well-versed in hair, too). I had some very unsympathetic (male) doctors so I'd given up on asking, but my primary care doctor is female and sympathetic.