This is a preview of what a Biden presidency will be like:

During the run-up to the election, his campaign called a “lid” before noon eight times in one month. The whole time he was basically shut in his basement, rarely venturing into the outside world.
The few times Biden did venture outside, he was hopped-up on stimulants and heavily reliant on teleprompters. None of that is going to change when he’s in office. It’ll probably get even worse.
He will be the President in Name Only, a figurehead occupying a ceremonial head of state role–similar to the Queen of England or the Emperor of Japan. The big differences will be the lack of actual royal blood, and the lack of universal reverence and admiration from the people. He will rarely make public appearances, and I doubt he will conduct much business with foreign heads of state when they visit.
The country will be ruled in his stead by a technocratic regency council. I wrote about this several months ago. It’ll likely be a triumvirate of Kamala Harris, Nancy Pelosi and Chuck Schumer, as well as many other career Swamp creatures (all unelected, of course) occupying posts on the White House staff, notably Ron Klain, who was just chosen to be Biden’s Chief of Staff.
This sort of thing has happened throughout history–for instance, when a king was too young to rule, or when a king becomes too old and incapacitated. The most notable example is probably King George III of England, who ruled from 1760 until his death in 1820, but by 1811 due to old age and failing health, both physical and mental, George III was no longer fit to rule, and so Parliament passed the Regency Acts which allowed Prince George to effectively rule England in his father’s stead. Prince Regent George ruled for nine years, until George III died, upon which point the Prince Regent became King George IV.
The difference is that America is not a monarchy, and Joe Biden will be entering office already unfit to govern.