@Natalia found a "fun read" in #437593:

Verifying signatures
The Qubes OS Project uses digital signatures to guarantee the authenticity and integrity of certain important assets. This page explains how to verify those signatures. It is extremely important for your security to understand and apply these practices.

What digital signatures can and cannot prove
Most people — even programmers — are confused about the basic concepts underlying digital signatures. Therefore, most people should read this section, even if it looks trivial at first sight.

Digital signatures can prove both authenticity and integrity to a reasonable degree of certainty. Authenticity ensures that a given file was indeed created by the person who signed it (i.e., that a third party did not forge it). Integrity ensures that the contents of the file have not been tampered with (i.e., that a third party has not undetectably altered its contents en route).

Digital signatures cannot prove, e.g., that the signed file is not malicious. In fact, there is nothing that could stop someone from signing a malicious program (and it happens from time to time in reality).

The point is that we must decide who we will trust (e.g., Linus Torvalds, Microsoft, or the Qubes Project) and assume that if a trusted party signed a given file, then it should not be malicious or negligently buggy. The decision of whether to trust any given party is beyond the scope of digital signatures. It’s more of a social and political decision.

Once we decide to trust certain parties, digital signatures are useful, because they make it possible for us to limit our trust only to those few parties we choose and not to worry about all the bad things that can happen between them and us, e.g., server compromises (qubes-os.org will surely be compromised one day, so don’t blindly trust the live version of this site), dishonest IT staff at the hosting company, dishonest staff at the ISPs, Wi-Fi attacks, etc. We call this philosophy distrusting the infrastructure.

By verifying all the files we download that purport to be authored by a party we’ve chosen to trust, we eliminate concerns about the bad things discussed above, since we can easily detect whether any files have been tampered with (and subsequently choose to refrain from executing, installing, or opening them).

However, for digital signatures to make sense, we must ensure that the public keys we use for signature verification are the original ones. Anybody can generate a cryptographic key that purports to belong to “The Qubes OS Project,” but of course only the keys that we (the real Qubes developers) generate are the genuine ones. The rest of this page explains how to verify the authenticity of the various keys used in the project and how to use those keys to verify certain important assets.

-- qubes-os.org, Verifying Signatures

Very good information about digital signatures and the involved trust. That's why I will pin this comment.

Big probs to @Natalia for finding and sharing this link!