Where does Google Tasks tasks arrived from? 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.

Examples:
Google tasks has been since long, but only in Gmail. Here is a 2009 article explaining it. 
I guess it came around the same time probably. There wasn't a separate app for it back then. It was just a chat-type window popup in the bottom of your gmail screen. Check the old Gmail, the tasks' list comes from the drop down in the top left (circled red)