The correct answer is:

“What for you are the giveaways that they are not native speakers?”

Their native language is the same as mine, so I presume I’m just sufficiently attuned to its sound to hear its influence peeking through. However, I suppose that the main giveaways for native English speakers might be the following ones in particular: No. 1 is mixing up several British accents, No. 2 puts too much emphasis on dropping the T and there is a certain Continental European hardness to her consonants, No. 3 devoices consonants at the end of words and her pacing might seem unnatural.

Everyone’s account is 1 day old at some point. And only one of those clips is even remotely business-related. My general question is purely language-related - the specific contents of these clips are purely incidental.

“but I will say blended accents are so delicious to my ear”

Yes, they sound unique, hence I’m interested in the question of the degree to which they can be decomposed into their individual elements in the ears of linguistically sensitive listeners. Insofar as they can’t - which seems to be the case here, at least as far as you are concerned - they can be said to add an authentically novel quality to the phonetic repertoire of human speech. Also, I have an impression that, unsurprisingly, they tend to be particularly clear and crisp, since they are used by individuals who consciously want to be understood. As a result, the phonetic features of their constitutive languages tend to be articulated very distinctly, which is a pleasure to listen to.