The way the web works, any CA (Certificate Authority) can create TLS certificates for almost any website.
This means that the people with these keys could MITM almost any https connection people make, not only the ones that this CA issued.
Yes and NO. These are intermediate CA's and for them to be recognized by your browser or system, they need to be signed by another CA or root certificate that your system already trusts.
SO, not all CA's are signed by root certificates already on your machine
"any CA (Certificate Authority) can create TLS certificates for almost any website."
This has always been incredibly stupid.
It doesn't have to, you can set X509 name constraints which say "the CA may only issue/may not issue certificates for these Subjects," or you can set CAA in DNS, for example.
(Of course, the former isn't really intended to constrain trusted Root CAs, moreso internal PKIs. But it is an option people should know about!)