I recently installed Google Tasks and was surprised to find tasks and lists over there. I haven't used Google Tasks on the web (or from gmail) either.
Those were old tasks/notes taken around 3-6 years ago.
I checked my Google keep, Google reminders/assistant and Gmail (which are the platforms I use and used for reminders/notes/lists) and haven't found any origin for them.
(I rarely delete, I always archive, so if it existed in one of them it should've been found).
Where did they arrived from?
By beeshyams' advice is I searched in my Google activity for phrases and tasks I had in the list, but I only found them in the search history, and not in any kind of task/list/note app.