How are professors able to multitask? How are professors able to multitask seemingly well?
I see some professors are able to work on their laptops in seminar or other presentations, sometimes talk to the person next to them, and then still come up with a suggestion or question at the end of it which bemuses me.
As in many situations, more novelty creates greater cognitive load, while non-novelty creates essentially no load. A teensy bit of interesting novelty is little cognitive load, but/and can be quite provocative, depending.

Since relatively experienced people have ... experienced many things... few seminar talks would present much that is radically new, even if it is new to novices. So, even if it is not somehow made explicit by the speaker or by the senior members of the audience, 90% or more of any talk will be a recapitulation of known things... Yes, it is understood that this is mostly necessary, to set a common context for discussion, and that there is not any genuine novelty there.

Also, there is a self-referential aspect to the premises of the question. Namely, the people one sees multi-tasking happily are the ones who can do it. Those who prefer mono-tasking will not be seen multi-tasking. So there's a "selection bias", as well.

And, for me, as for many relatively senior people, I/we have seen many things, and even superficially novel aspects of a seminar presentation may be echoes of things we thought about 30 years ago, etc. Not that there's not progress, but that there are questions that have been pending for decades, and preying on our minds, so (hoping that our minds are still working well) it is not burdensome to hear contemporary progress, but, rather, a happy event. But, in contrast, novices of course have no reference to related technicalities they reflected upon 30 years ago, etc.

And, apart from a greater accumulated experience of ideas and facts, I imagine that a vastly greater experience of coping with novelty and confusion gives a great advantage. E.g., if a smart kid has never in their life before been seriously confused, it is a disturbing experience. And this is a serious issue for very-smart kids in most environments. Similarly for very-smart kids in most undergrad environments. Even up to a point in graduate programs... but/and all the worse to soooo belatedly (after establishing habits that cannot accommodate for this) discover that high-level science (etc.) involves a constant rain of confusion and exceeding-parameters. (For me, this is entertaining, yes, but for those who think that people can control/understand everything easily, and not be confused, it is often deeply disquieting, I gather.)

Practice makes perfect.