MyKeyFinder scans the registry and lists what it thinks might be keys, along with the name of the software that key belongs to. It doesn't list every key, since not every software key is accessible in the registry. Some of the keys that are listed are for software that doesn't require a key, e.g., AMD drivers. The keys themselves may or may not be accurate -- some software stores an altered version of the key you used, plus software like MyKeyFinder can't know the difference between a legitimate key and some other value. It can still be useful if you can't find a record of a key, or if/when you have a device that came with Windows already installed, and will print a list of selected keys or save it to PDF.
A quick note for Windows keys: the reported key for Win10/11 may not match the key you entered, and may or may not work to activate another copy of Windows. Activation is tied to a secret hardware key that's derived from the components used in that device -- in theory you should never have to re-enter the key as long as the majority of the hardware remains the same, with the exception of the network adapter -- changing the MAC address can cause deactivation. You *may* be able to remove a key from use with one device and use it on another depending on your Windows license, but that doesn't always work according to reports. Microsoft just recently turned off using Win7/8 keys for Win10/11 -- something that officially ended in 2016 -- so they *may* be getting stricter about keys & activation.
MyKeyFinder adds the program's folder along with a folder in Users\ [UserName]\ AppData\ Local\. The majority of new registry entries is for uninstallation, though it also adds a handful of fonts.
tomcat posted in the comments about an alternative free app called LicenseCrawler -- I wanted to add a quick note on that software... it looks like it can provide the Windows key, though with a few Windows keys listed you might have to do a bit of trial & error. Deciphering what key goes to what app might also be challenging. The *portable* version has a nag screen, and uses the older visual basic and active X, which are not often seen nowadays -- because of that I recorded 3607 new registry entries in my Win11 VM.
Otherwise we've had a few giveaways from XenArmor, who makes key finders, Nirsoft [nirsoft[.]net] has the free Produkey, available as a real portable app that gives you the Windows key -- Nirsoft has a Bunch of free utilities, including several that *may* allow you to find additional, encrypted keys -- and this article on Tom's Hardware shows how to get the Windows key on your own, no software needed.
tomshardware[.]com/how-to/find-windows-product-key