Literally dozens of articles were written about a red mirage and how the mail in vote which would be counted later, would skew towards the democrats.

I think it's mainly stuff like this: https://i.imgur.com/3pvKeLG.png

A skew is one thing seeing where one line eventually overtakes an other. However, discontinuities like that image definitely should be explained. The image above I think already has explanation. However, for some time after the election no explanation if I recall. So if one is seeing things like the above with no reasonable explanation, and they were already unsure about the about the election's integrity. It's pretty obvious to see why things like this would rile people considering the atmosphere leading up to election day.

In one case a collection of votes was delivered that contained over 400k votes for Biden and zero votes for Trump or the independent candidates. That is incredibly improbable verging on statistically impossible even if 99% of those mail-in voters supported Biden.

Cite from a reputable source on this? I know there were stories of related things happening, but to my knowledge they were all bullshit.

Why is the opposition to blame on this? There were dozens of articles written to explain the effect. Contrary-wise, why didn't their leaders explain the "unprecedented anomalies" themselves?

What exactly would the opposition have to have done for you to not say they made no effort to explain?