According to some Constitutional scholars and lawyers, impeaching a private citizen is UNCONSTITUTIONAL.
The Articles Pelosi and the House passed have not yet been delivered to the Senate, thus the impeachment process was not complete before Biden was sworn in.
The Constitution gives the House and Senate the power to impeach and try the president, not a former president.
Nixon was obviously guilty of an actual crime (obstruction, etc.). If his resignation wasn't going to protect him from impeachment, then why did he do it? In fact, the Articles from the House had already been passed and transmitted to the Senate, but the Senate wasn't able to formerly begin their process before the resignation. So, the Congress had everything in motion, almost exactly as the situation now has been started. Yet, they didn't try him after leaving office. Ford's pardon of Nixon was to prevent a civil or criminal trial and wasn't offered immediately, so the pardon wasn't stopping the Senate.
Fast forward to 2024. If the Senate actually convicts, and the Congress wants to stop Trump from becoming President again, he will have a chance to challenge the impeachment in court, which I think he'll win. Right now, Trump is not under the jurisdiction of the Senate.
Lastly, an impeachment is a POLITICAL process, not a criminal one. There is only one remedy if found guilty -- removal from office. To pursue impeachment now is fruitless.