Is obstructing Congress the same thing as obstructing justice? I thought the judiciary branch had to get involved for the later.
No and no. But here is why obstruction of Congress should interest even someone like you:
Suppose what Democrats have accused Trump of were true (abuse of power or bribery to smear a political opponent in order to increase his chances to win the next presidential election), suppose he really did freeze military aid to Ukraine to coerce the Ukrainian government to manufacture dirt on Biden, would it not be important to provide Congress with the means to investigate such actions and hold the president accountable?
For if a president can (ab)use his office to coerce foreign interference in US elections, he can, given US power overseas, virtually buy himself re-election. Would that not, for all intents and purposes, make him a dictator?
Now, the Founding Fathers, rather liking the democracy they had set up, thought that if a president were to undertake such actions, he would need to be checked. That is what impeachment is for. That is why the Constitution gives Congress the 'sole power' of impeachment (the House to impeach, the Senate to try impeachments).
What Trump has done, whether guilty or not, is to deny the House the means to investigate his actions. Disobeying the Constitution, he pretends and has declared categorically that the House has
no authority to investigate his actions. Had not some former and current officials from the administration defied his order to keep quiet, there would have been no witnesses, no documents, no evidence - except for what became public through other channels. In effect, he would have shielded himself from accountability. He could go on coercing foreign interference and buy himself another term.
It is due to this cover-up that the evidence presented in the hearings, damning as it was, was not outright 'outlandish' (the GOP House counsel's bizarre standard for bad conduct).
Of course, the House could have fought the administration in court to force the testimony of witnesses and the production of documents; but that would have taken about a year. During that time in court, Trump could have (and, let's be honest, would have) continued to solicit foreign interference to win re-election.
So whether or not you think Trump did something wrong regarding Ukraine, he definitely violates the Constitution and threatens US democracy itself by obstructing Congress. Pretty bad, considering he swore an oath specifically to "preserve, protect and defend the Constitution of the United States".