The safest approach is to treat them like any other personal media archive:
Store the videos on a dedicated folder or drive, rather than scattered across Downloads and other directories.
Use a consistent naming format, such as Channel - Video title - YYYY-MM-DD.ext.[/ICODE]
Create subfolders by channel, subject, or year. Avoid making the folder structure excessively deep.
Keep a text or spreadsheet index containing the title, channel, original URL, download date, and any notes.
Use a standard format such as MP4 with H.264 video and AAC audio for broad compatibility, unless you have a specific reason to use another format.
Maintain at least two copies: one working copy and one backup on a separate physical drive.
For important material, keep an additional backup offline or in reputable cloud storage. A backup connected to the computer at all times can be affected by ransomware.
Periodically open a sample of files and check the drives for errors. A copied file is not necessarily a good backup unless it can be restored.
For a large collection, avoid relying only on Windows Explorer search. A cataloguing application or a simple CSV file makes it easier to find videos even if filenames later change.
Only download videos where you have permission or where downloading is allowed by YouTube and the content owner. YouTube’s terms and copyright restrictions may apply, and locally saving a video does not necessarily grant redistribution rights.
The key principle is the 3-2-1 backup approach: three copies, on two different types of storage, with one copy kept offline or off-site.