Pictured: Not the drive I just picked up

So I do my research before coming here, but nonetheless when I have issues with tech, my car or other things, I like going here because I like actually talking to people with some experience, which this place has proven to have. Anyway, I got the replacement drive for my laptop and it’ll be here soon. Yay. Now I need to get Windows. Boo. My laptop didn’t come with a disk to re-install Windows or anything, nor any codes. On my searching around I found that you can purchase Windows 10, the OS my Laptop came with, for about $100-$300 bucks. Yeahno. On eBay and other places they’re selling Windows 10 “keys” for like $10. From what I’ve gathered these keys are like a subscription key for when you buy an antivirus program or other software. Thing is, how would you even install Windows with a key on something without an OS in the first place? And if any of you have experience with keys for WIndows, will it be a one-and-done deal or am I gonna have to keep throwing cash at this just to keep my Laptop’s OS?