How to ensure that the data cannot be recovered after the computer hard drive is formatted?

Because I have experienced the test program source code that took several years to write and was stolen by someone else through the host hard disk recovery data after resignation.
So I found this article in particular and try it out next time.
①Copy the prepared oversized file to the formatted disk until the entire hard disk is filled, which is time-consuming, so please wait patiently;
②Full-format-full-format, eachDisk more than 3 times;
③When formatting, select the allocation unit size with different values;
The last step: Reinstall the system on C disk, delete it completely, and keep the data from being recovered forever.

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