> 8,620 respondents, but I don't see how they were recruited

This is from the PDF listed on the page that you linked:

    To establish a nationally representative panel, members are recruited on
    an ongoing basis through address-based probability sampling using data
    from the US Postal Service’s Delivery Sequence File. Recruited adults in
    households without internet access are provided a web-enabled device and
    free internet service. A probability-proportional-to-size procedure was
    used to select a study-specific sample. All panel members who were aged
    18 years and older were eligible for selection. Invitations were sent by
    e-mail; automatic reminders were delivered to non-respondents by e-mail
    and telephone beginning 3 days later.
    
    A final survey weight variable provided by Ipsos adjusted for the
    initial probability of selection into KnowledgePanel and for
    survey-specific nonresponse and over- or under-coverage using design
    weights with post-stratification raking ratio adjustments. With
    weighting, the sample is designed to be statistically representative of
    the noninstitutionalized adult population of the US as reflected in the
    2021 March supplement of the Current Population Survey.