It's complicated but I will try to explain. The portal exist or come into existence when the time machine is turned on, and stretch only in one direction. Forward in time by a fixed amount depending on the power used. Effectively becoming a bridge that moves in sync with the progression of time. So let's say you turn the time machine on at midday 8 th of January 2021 and you create a portal to the 8th of January 2071 50 years from now. As the days progress from the 8th to the 9th to the 10th to the 11th 2021 the other end of the portal progresses from the 8th to the 9th on the 10th to the 11th 2071. How do we know where the end of the bridge leads in time. Somebody steps through the portal as a scout. This is how the original time machine worked. But the traveller couldn't return back. Go forward but no return. Until a electrical engineer developed the B Array that changes the direction of the temporal wake within the portal by bombarding it with tachyons. There are numerous limitations as long as rules that have to be adhered to. You are limited to the amount of times you can travel through portals. This is not an arbitrary rule it affects your physiology and cognition. You can only travel to times where time machines have been used. Farthest we can travel back to is the 1920s. Zero points the invention of time travel. There is only few portals that bridge to a few points in time. If we need to go to a specific point in time we have to utilize another method, stasis. Temporal agents which is what I am, don't bring back any technology. The minute you got back into time no matter how careful you are you change the timeline from whence you come. The further back the greater the change and the more careful you have to be. If any future technology ended up in the past it could create a paradox, knowledge from nowhere, but could result in catastrophe change. The future I come from is a dystopia but at least it better than no future . As for the food in the future it's just bland crap, lab grown burgers etc. Your future my history, coronavirus will least 2 to 3 waves just like 1918 Spanish flu . New strains of the coronavirus will appear and be with us for many decades to come with many more lockdown, that will mainly affect people with comorbidities. But a variant of coronavirus will emerge from the Asian subcontinent which will affect male fertility.