tls – How does ssl decrytors works? Is it a man-in-the middle?

I am studying for Comptia security. I learned new technology called SSL decryptors. I still cannot get my heard around it, for me it just seem like a kind of man-in-the-middle (on-path). Basically it just sits in the middle of traffic, decrypt the traffic, inspect for malware and encrypt and send.

1 – Since ssl/tls is a secure technology, where is ssl decryptors getting private key from ?
2 – Can it decrypt traffic every website ?
3 – Can this tool use for malicious purpose ? (sitting in the traffic and capture traffic users/password)

Thank in advance.