I asked this question to answer it as I was not able to answer to a similar question(Search only subscribed channels in YouTube) as it is a highly active question and my reputation is low as I have just opened an account today. I thought it would be effective so decided to take this step.
This is one of my first answers just copy-pasted. So, please excuse the momentary dips in quality of this. Also, I am not a programmer but found this effective method through Trial-and-Error. I have put this answer on several similar questions as I do not really know if there is any reshare option.
I do not know if you got the answer already or not but this is something I found out just today.
Now, All of these require chrome extensions for pseudo-Whitelist These are the links. Use the group policy for making the extensions admin force-installed as it can seriously mess up willpower as it will feel as if it is better to uninstall the extension instead😅
DFTube(Optional) - https://chrome.google.com/webstore/detail/mjdepdfccjgcndkmemponafgioodelna
Channel Blocker -https://chrome.google.com/webstore/detail/nfkmalbckemmklibjddenhnofgnfcdfp
uBlackList (Optional)- https://chrome.google.com/webstore/detail/pncfbmialoiaghdehhbnbhkkgmjanfhe
Download these and go to DFTube - You can completely remove recommended tab on youtube if you wish along with others in the case it distracts you - It's optional
Now come to the main one - Number 2 Channel blocker - In this, Click the Options of this extension after installing it. Click the "Advanced View" option then in the dropdown menu, select "Block Users/Channels by Name" and then in the box write all letters individually from A to Z. such that in the block list it shows all letters of the alphabet(like "a" *,"b" *, "c" * on each line and so on) IMPORTANT: BEFORE DOING THIS, Write a list of all the youtube channels you only want in your recommended or even your youtube search results. Now once you get only the list of youtube channels you want, then click the dropdown menu again and click the option asking " Exclude these users/Channels" and write the TITLES of these channels down individually with PROPER SPACES IN PLACE.
Now scroll a bit down in the extensions options and in the "VISIBILITY" option turn the "Block-animation speed" to "fast" by dragging it towards "fast".
Now whenever you shall try to access any channel other than the one you explicitly put in the exclude section WITH PROPER SECTIONS, then all the other videos shall be block very fast - including the channels and users (Except Channels/Users with Language characters other than English like Japanese and Chinese)
As for blacklist - Again it is optional as it will remove youtube search results from google.com, bing.com, and other engines. If you are ready for it - Here it is:- Just after installing the extension search up youtube - You will find a "Block this site" button appearing which you can press to block any website (youtube in this case). Now AFTER THIS IS DONE go to the extension options for uBlacklist - Find the option "Hide the number of blocked sites and the "Show" link" and turn it on. If you use search engines other than google - enable all of them just above the option which I mentioned. So in case you want to search a particular channel - it shall be searchable only on youtube.com and no other engines.
Now, All of these will work together and put only certain websites on your recommended/search feed and block all channels except the ones in the "Exclude List". Now, if you are putting these off for distractions - Use a tough blocker like Cold Turkey or FocusMe to block the option to change the extension's options by putting the extension ids in blocking urls by keywords option.
Boom! Youtube now blocks rest of the channels very fast and also blocks Every user/channel other than ones you see alone and you can get some serious-serious bang-your-head-on-the-wall productivity😂🤣
If all of these don't work just use another extension like Blocksite or Stayfocusd to block the urls you do not want !