When I push one task with its row button, which app does it go to?

To every connected task app that's switched on for pushing — filtered by that list's push destinations. Each list can be set to push to only certain apps, and the row button respects that: apps the list isn't routed to are skipped silently, as is any app that task has already been pushed to. So a single "→" might create the task in Todoist, Apple Reminders and Google Tasks together, or in just the one app its list is routed to. The status line tells you exactly which apps it reached (or "Already pushed ✓" if none needed it). Tasks in a padlocked list never push anywhere.