Two switches in Settings control this, and both are reciprocal: whatever you hide of others is hidden of you too.
Show my activity is off by default. While it is off, nobody sees your "Seen" or last-online, and you do not see theirs. Turn it on and both directions open up, with last-online always a coarse phrase, "recently", "this week", never a timestamp. Read receipts are on by default; turn them off and you neither send nor receive them.
There is no way to watch others while staying invisible yourself. That is the point.