Spoiler for Latest Discovery short take episode.
How is the prime Directive used in Star Trek? As a writers tool to manufacture drama, lol. I can't think of an instance where it stopped anyone from doing anything. If anyone has an example of prime Directive being followed please share.
As for the last Discovery short take. If the Kelprians (?) Had seen ships only, using transporter might be a violation. The species take Saru's people aren't in Federation, so prime Directive doesn't apply to them. The idea that the federation is the galaxy's morality police is both unexplored in trek and a very disturbing idea. Flying telling everyone what is or isn't allowed sure seems culturally respectful, not really. It is a great way to make conflict however.
Janeway ends up having it turned on her in Prime Factors, which I always found interesting. Is there anywhere the PD is spelled out (by show writers or Rodenberry etc)? I do not remember. It always seems to be "more of a guideline" (yes, for writing purposes). Perhaps Lt. Georgiou's argument to SF for retrieving Saru was similar to Kirk in Return of the Archons in that because of this harvest agreement, the Kelpians are not a "living and growing' culture (as they are not. The entire basis of the arrangement appears to be to keep the "balance", code word IMO for "status quo").
We're going to have to do a special episode just about the Prime Directive someday. I've never bought into it -- I don't understand what the benefit is to anyone involved. If the Federation has the ability to cure cancer and provide technology that solves the problem of resource scarcity is it moral to withhold that in order to protect the potential recipients' culture? How does the Federation even define "culture" in this context? This has always struck me as similar to claiming that the reason I won't help solve the problem of homeless veterans is because they have a unique culture and I don't want to interfere with its development -- a position I don't think anyone would find morally just. I'm not sure the Federation really acts as the galaxy's morality police. In fact, I think the Prime Directive is set up to precisely to prevent the Federation from acting in that capacity (even if we see our captains breaking the Prime Directive without consequence ALL THE TIME). Otherwise, the Federation would intervene here to protect the Kelpiens from being enslaved (or eaten?) and would likewise put an end to the Orion slave trade. I don't know if we have any instances of the Federation facing the choice of preventing a third party from perpetrating a genocide, but my sense is that the Federation would let it happen -- that the Federation would have sat out the Second World War and the Kosovo War. Indeed, the Federation strikes me as Calvin Coolidge's utopia -- all Federation citizens are rich, and while the Federation looks around for new opportunities to incorporate other like-minded societies into its cultural and economic world and actively colonizes uninhabited planets, it is otherwise isolationist regarding the imperial powers or any other civilization that has different cultural practices.