In my neck of the woods: phone only for the vast majority of people.
Even people who use a computer professionally at work don't use one at home.
The only people with home computers I know of are millennials who grew up with them. Younger and older simply don't have computers at all in their lives. Nobody can type anymore.
Monitor and keyboards won't disappear, but it will become more niche with time. It won't be a universal experience like it used to be.
Not every part of the world has a tax office work ritual. In my neck of the woods they don't.
And if we are talking about the paperwork heavy countries, there are tax offices in abundance where people come in and have a third party type their info into a computer... could they have done it on their own? yes.
And I'm telling you that I literally do not know anyone who has a smartphone but not a laptop or desktop. That's probably because phones can only ever achieve so much. Try editing spreadsheets; even Google Sheets is severely limited on mobile, much more so Collabora Office.