// Why, despite Covid and everything else, a country as "advanced" as the USA can't count the election results in one go within 24 hours is plainly ridiculous. //
One reason is that not all of the votes arrived on election day itself. For example, overseas ballots (and military ballots) can be counted for several days after the election (rules vary by state). Another is that the practical exercise of counting postal votes is lengthy. Each envelope has to be opened (usually twice, because often they're meant to be sent inside two envelopes including an internal "security" envelope), and then the signature checked against records etc. Given that the volume of postal voting this year was far greater than previous years, this naturally drags the process out, especially in states where postal votes weren't even allowed to be counted until after polls closed.
All of this drags the process out, but normally that's fine. In 2016, for example, the result was "called" on the night, despite some millions of votes still to be counted. Back then, for some reason, there were far fewer complaints about the delay, or the process, from Trump et al.