Under Covid-19, many countries don’t allow visitors in. So I wonder whether it would be possible to travel from transit zone to transit zone, without a planned itinerary. Say, I book a ticket from Hong Kong to Taipei, then stay in an airport hotel. After a few days, I find a cheap flight to Tokyo, fly there, etc.
While the concept is interesting, it is unlikely except perhaps in very limited cases.
The issue is that it is difficult to prevent you from leaving an airport. So, when boarding, you would have to meet the requirements of the destination country of your ticket. Simply telling the agent that you don't intend to leave will not make them waive pre-boarding verification. This will apply to each ticket and so you will quickly run out into an impossible scenario.
One frequent restriction is to have an official COVID-19 test without a certain time frame before boarding. On your first ticket, you would have it and it could be valid for one or two more, depending how far you travel but eventually you would need to get another test to board the next flight without leaving the secure area.
There are flights to nowhere that depart and return to the same airport but those would not count as traveling the world, not even traveling, I would say, just flying.
So I wonder whether it would be possible to travel from transit zone to transit zone,
When boarding a flight from A to B, the airline MUST check whether you meet immigration and Covid requirements for your destination country and will decline boarding if you can't show these.
If you have a "good" passport you may be able to get away with the immigration issue, however Covid testing will not work. Many airports these days do have Covid testing facilities but they are almost all landside, so you can't stay in the transit area (even if they let you).
This is not practical because many airlines will check if you have the right to enter your destination country. In theory you could book the entire trip before leaving, and claim that your destination country is the same as your home country, but your question was about booking onward travel when already partway through the journey.