Storage drives

Greetings all. I've searched around the net, MS, various pc sites, and here and am getting a bit frustrated with the overwhelming amount of useless or non pertinent information that I seem to come across when researching, well, pretty much anything these days. It's hard enough to get barbeque sauce for my burgers anymore, let alone try to find some decent information about my computer. So after a few hours without much more info than I have on my own, I'll just post a question and see how it goes. I'll start off with some info about my laptops and then let it rip. I have an older Dell Inspiron 15-3567. It's been a good computer but I decided I wanted a 360° hinged 2 in 1. I did some shopping around and ended up with a custom Inspiron 15-7586. It's got the Core i5 8265U, 24gb RAM, 256gb SSD, and a 2tb SSHD storage drive. I love it. I kinda wish I'd gone to a store and looked at sizes because I probably would have opted for a 14 or 13.6". The 15.6 is a bit big and heavy to really use as a tablet. I can't believe the have 17.6" models, phew! But regardless of the size, I do still love it. My old laptop is still in great condition and I have a lot of data and info on it, so I decided that since I got such a steal on my new one that I'd put a bit of money into it and keep it for a work pc. It was stock with the Core i3 7100U, 8gb RAM, and 1tb HDD. I really like the SSD and dual drive in the new one so I look into things and see that the 3567 doesn't have a dual storage drive option however looking a bit further into things I discover that I can swap the optical drive for a caddy mounted storage drive, and that's exactly what I did. So I got a second matching RAM unit to bring it up to the max supported 16gb RAM, I put a new Hynix Gold 500gb SSD in the C: drive spot and put the 1tb HDD into a caddy in the optical drive bay. I also put a new coin cell battery on the mobo, upgraded the 2.4ghz WLAN card to a dual channel 5ghz so I could use the bandwidth of my wifi and network with my other devices, put the optical drive into a USB 3.0 enclosure so I can still use it. I bought a new genuine Dell main battery even though the original is still in good health. It is 3+ years old and is down about 5% as far as how it holds a charge compared to when it was new. I had a little hiccup with the old drive as I assume that when I upgraded from Win 10 Home to Pro a short time ago, it must have encrypted the HDD with Bitlocker, but once I got logged back into my original Microsoft account, I found the key to unlock it. Now my issue is what to do with the old installation of Windows that's still on the HDD as I just did a clean install on the new SSD. I have most of my data as far as media, movies, music, pictures and what not separated out but I'm sure there's some stuff that's still embedded into program files, program data, etc. I can muster through there and extract that stuff manually if need be, unless anyone who's still reading this knows of an easier way? But aside from all that, What do I do with what's left of the old Windows installation files? Just go through and delete them? or is there an uninstall protocol? I don't know, I haven't had to do this before and I couldn't seem to find any decent info on it. Anything anyone can throw at me regarding this is welcomed and appreciated. Thanks if you took the time to read my small novel, and happy Delling.