So there are kinda 3 answers to your question:
Who owns Lemmy? Nobody; it's FOSS
Who owns Lemmy.ca? Smorks
Who owns activitypub protocol? The world wide web consortium created it I believe, but it's an open standard and will likely evolve based on which organizations use it. In the same way as how HTML and HTTP have evolved over time alongside the growth of some of the largest applications that use those standards.