Yes, hidden lavaliers should be an obvious answer. I somehow didn't think they would go to the same lengths of hiding the mics as in film productions.
When I worked on a few TV productions, I know we were hiding the mics in pockets, scarfs, hair, neckties, hats, taping to the chest, then planting mics in different props, flower pots, taping under the table, behind some objects, etc.
I also thought that sound quality consideration would be higher - in the film it's usually a short scene that can be repeated - and if some mic malfunctions or some jewellery or fabric makes too much noise, or the actors hug and press against the mics, etc. it can be repeated and amended...
High level of planning and expertise goes on here, obviously. Also mixing wise. It would be interesting to hear an interview about it from those guys who did it.
I have watched many opera productions with decent, adequate, but "distant" sound of the soloists - with mics visible at the edge of stage. Like this:
https://www.youtube.com/watch?v=GRjp...uj51pV&index=6
While the one I started this thread for sounds almost like an album recording.
So, it is hidden lavaliers after all, what else.
I now spotted one peeking out from the edge of the jacket of the baritone singer in a few scenes - see the attachment photo - it's the exact spot we would put it when actors had jackets in the TV show I was working on. For female singers with the bear shoulders and no cable going up to the hair, I guess it's the cleavage...

Taping to the chest was usual, too. But also problematic for unwanted noises... cleavages were fine mostly, since the mic capsule was open/unobscured. Under the shirts, etc. was a bit tricky, especially with synthetic shirts...