Registry is mess. How can I clean registry in windows 10?

I have mixed opinions on this.

As I understand it the original "registry cleaner" was a software development tool, used to ensure the machine used to build a software package was in a clean state so that unwanted registry keys wouldn't be captured along with the build process. I might not have it exactly right but I can't point to the page I read it on now, it was years ago.

So officially you don't need one.

Unofficially I've had a Windows 95 machine with a slightly broken registry, at the time I exported the entire registry as a .reg file then re-import this. DO NOT TRY THIS NOW as I think from XP onward it is more complicated and database-like, but at the time it flushed out something bad.

I've also had problems with badly-uninstalled shell extensions. In theory the leftover keys shouldn't do anything once the files they refer to are gone, but that wasn't my experience.

Another problem though possibly not registry was a reference somewhere to a file on a network drive. This caused a long startup delay waiting for the file to time-out.

More recently a 64 bit software package I'm installing appears to include a chunk of files from an older 32 bit version which takes over some file extensions. An automatic cleaner didn't help though, direct intervention is needed.

Frankly programs that take over file extensions in a heavy-handed way are one of the main arguments for registry cleaning.

Then there's the two machines that spam command windows when they start up. I'm not aware of a malware cause, it appears to be the result of a telephony software package where the startup launcher was launching an app indirectly via CMD.exe instead of just running it.

As to what to use I understand that one of the most popular ones may have gone through a change of ownership, so personally I can't recommend it right now.