PKI: self-signed digital certificate?

What is self-signed certificates? Can you make them free? Are self-signed certificates less secure than those signed by commercial CAs. What is the difference between commercial certificates and self-signed certificates. What situation suits self-signed certificates or commercial certificates?
You will find all these answers in this video.
Playlist: Advanced Cryptography -
• What is digital signat...
Playlist: Basic Cryptography
• Private Key Encryption...
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