I usually switch my brain off after hours, but for example, recently at a baby-party event, someone was asking what I do for work. Ended up asking if my company would like to bid on their project, gave me a bunch of insider info.

I don't want to be that person who only talks about work, but small comments here and there, like planting seeds, and then you get emails of "can I introduce you to X, who is looking for Y?"

Goto meetup.com or any event/meetup website. I found this Python ML meetup group (could be any technical group), joined, and went to their weekly weekly conference (more than a hundred in attendance). I'd sign up to go up the stage and talk for a couple minutes just introducing myself and what I did. After I got off stage, interested people would come up to me and chat with me. There definitely had been a few collaboration propositions.