I suspect from the states' Websites - I know California has one.
According to this post large news companies such as Reuters and AP arrange for local residents to go and get a count of election results from clerks, local officials with access to the count etc. Two or more people independently write down & report the data. Some news organizations may have access to officials that work for election commissions or local news feeds with access to authoritative sources and use their access to cross-check the reported data.
Many smaller news sources and Google (which has access to various online news sources) source their data from these large news companies. The data compiled by the local residents that has been cross-checked now goes on the news companies' sites and appears on Google.