With over 81 million votes, Biden received the most votes of any presidential candidate in history. It is also true that he won a record-low number of counties – but counties vary by population size, so those wins don't correlate with the popular vote.
People vote, not places, and Biden won the most populous counties in the country, according to a Census Bureau report. It's also disingenuous to suggest Biden's small campaign events indicated a lack of support, given the ongoing viral pandemic.
The Trump campaign disregarded many COVID-19 precautions and hosted large crowds, while Biden made following coronavirus guidelines central to his efforts. Biden's lack of crowds does not translate to a lack of support from voters, but rather a consistent adherence to pandemic safety guidelines.