How to search subfolders in Finder

Have you tried re-indexing Spotlight? Spotlight- How to re-index folders or volumes - Apple Support or do the following from the Terminal:

Force Spotlight Re-indexing

Open the Terminal in the Utilities’ folder. Copy each line then paste it at the Terminal’s prompt and press RETURN. You will be prompted to enter your admin password. It will not echo back to the Terminal window. Press RETURN again. You should only be asked when you run the first command.

     sudo mdutil -a -i off

     sudo rm -rf /.Spotlight*

     sudo mdutil -a -i on

     sudo mdutil -E

This should cause Spotlight to begin indexing the drive. During this time you may find the computer is slower than normal. This is OK. Don’t use spotlight or do searches until indexing has finished. This could take several hours so be patient.

Boot into Safe Mode (Use Safe Mode to isoloate issues with your Mac) and test the search function from there. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

Like Kappy my search searches the the folder I'm in and all subfolders.

Here's a search in my Documents folder that finds files 2 folders down:

Also have you run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your machine?

Sorry, but it searches sub-folders of the current folder on my system. Has worked as long as I can remember. Perhaps you should be looking elsewhere for a solution before you complain. Furthermore, if you have a problem with performance or user-friendliness, then state what the problem is. This is not a complaint site. Read the Apple Support Communities Use Agreement - Apple Community.

Please read Writing an effective Apple Support Communities question. We are here to help. We are all users. We are not mind-readers nor clairvoyants. We only know what you write down and tell us. Be specific and thorough. Include your hardware information and installed version of macOS.

FYI: These are user-only forums. You are not speaking to Apple. if you need to speak with Apple Support, then contact product and tech support:  Contacting Apple for support and service including international calling numbers.

Thank you for your understanding and support.

What kind of search are you trying to execute? When I want to do something like this, I use the find command from command line interfaces (CLI) in the Terminal application. Enter man find to get information on using it, and you will find a lot of examples on the internet. I have found that complex tasks are often better going through shell scripts and CLI.

It might be possible to do something with Smart Folders, but I haven't played with it all that much. I personally think that using the find command would be easier and more understandable.

The command find  .  -Bmin -30 -ls will find all files in the current directory and its subdirectories that are less than 30 minutes old.

I have used searches from the Finder which is what the OP is asking about. Basically, it's a Spotlight search which is probably quicker than the command line's "find" command unless it is also relying on a pre-indexed drive like Spotlight.

The OP's issue is one I think may be unique to his/her computer. I cannot replicate it on my system running Catalina.