It certainly is a mystery why legally cast mail in votes (which are allowed to arrive after election day per state law in some jurisdictions) would sway to one party. It's almost like one candidate has been telling his supporters to, in no uncertain terms, not use mail in ballots.
Your other claim is literal misinformation.
Philadelphia City Commissioner Al Schmidt, a Republican: "Observers from the Democratic Party and Republican Party, from the Biden campaign and the Trump campaign, have been in our counting area observing, right up against where the process is taking place, from the very beginning" (he said this in response to a similar lie by Ted Cruz)
Your other claim is literal misinformation.
Philadelphia City Commissioner Al Schmidt, a Republican: "Observers from the Democratic Party and Republican Party, from the Biden campaign and the Trump campaign, have been in our counting area observing, right up against where the process is taking place, from the very beginning" (he said this in response to a similar lie by Ted Cruz)