I used to have a colleague that would always nod to things in meetings, and make a humming/agreeing ("mhm") sound once in a while. I used to find it a bit weird, he probably didn't even notice it himself. But then I held a presentation once with him in the room, and started to love that little quirk of his. Could always just find him in the crowd and feel some encouragement.

As for telling speakers you liked their talks, another higher up in the company used to send me a DM and thank/praise me. My guess is he did it for everyone, but still it felt very validating as a junior, both to be seen and also encouraged. It's a bit like with kids, tell them they're good at something (even if not), and they'll try to be, heh.

But I've tried to take that with me, and send DMs to people, where it maybe normally wouldn't feel natural for me. But after a while it does. And it can be even for small things, like "I like how well prepared you were for that meeting, any questions they had you could answer". You'll bet that person is well prepared the next time we have a meeting as well.