If you've watched reruns of Perry Mason, you've heard the prosecutor, Hamilton Burger, complain of Perry Mason's legal tactics as being "incompetent, irrelevant and immaterial!" (See video below)
President Trump and the Republicans apparently don't remember or perhaps didn't understand the nature of Burger's complaint. More likely, they are hoping and betting that you didn't understand what Burger meant.
n. testimony, documents or things which one side attempts to present as evidence during trial, which the court finds (usually after objection by the opposition) are not admissible because they are irrelevant or immaterial to the issues in the lawsuit. Thus, trial lawyers often object with: "incompetent, irrelevant and immaterial," figuring that covers the waterfront of most objections. --Source: Law dictionary
As applied to a piece of evidence in trial:
"Incompetent" means the evidence is inadmissible under the established rules of evidence. Hamilton Burger was not casting aspersions on Perry Mason.
"Irrelevant" means that the evidence doesn't have any tendency to prove or disprove a contested fact in the case.
"Immaterial" means that the evidence lacks probative weight and is unlikely to influence the trier of fact.
The House of Representatives impeached President Trump for abuse of power and obstruction of Congress. Trump and Republicans are demanding that Joe Biden and his son Hunter testify as witnesses at Trump's impeachment trial. That is strictly a diversionary tactic not based on any legal precedent. In fact, legal precedent is against the Bidens being called as witnesses.
Joe Biden said he wouldn't testify "because it's all designed to deal with Trump doing what he's done his whole life -- trying to take the focus off him." Biden went on to tell reporters, "look, the grounds for them to call me would be overwhelmingly specious. But so I don't anticipate that happening anyway".
Fact checkers tell us that Rudy Giuliani's and Trump's accusations against the Bidens are false and entirely without merit. The guilt or innocence of either Biden is not the issue. It doesn't matter if either Biden is guilty of poor judgment, making an obscene amount of money or something more sinister and illicit.
In a trial the defendant isn't permitted to incriminate and prosecute other people in an effort to somehow exonerate himself. If a criminal robs a gas station and then is arrested and put on trial, it's not a legitimate defense for him to summon and accuse executives of the oil company that owns the gas station of polluting the planet, of price gouging or making exorbitant salaries---even it's true.
Likewise, Trump cannot launch a defense that is incompetent, irrelevant and immaterial.