IIRC, Pence tendered his resignation after Jan 6. There were a few posts about it here around that time. This would have allowed Trump to make an interim appointment because the Senate was not in session until the "inauguration." I'd speculate that the current sessions of congress and senate are illegitimate, so the temporary appointment continues.

No sauce, just theorizing based on the chatter at the time. Who got the nod is also purely speculative. My favorite is Pompeo, who ambiguously tweeted that he'd be transitioning into another role. But take with salt because afaik these details are all hypothetical.