It was written as a parody of the fairly corrupt Japanese legal system, exaggerated for both humour and gameplay reasons, giving us such lovely gems as:
They don't have manslaughter
It's never stated outright to my knowledge, but it's generally implied that the penalty for murder is universally or near-universally the death penalty
Trials are legally mandated to go on no longer than three days, no matter how complicated they can get. The lab analysis for a poison isn't completed in three days? You can't use it in the poisoning trial. Your witness can't be tracked down on the last day? We go to the verdict without their testimony.
Everyone is assumed guilty until proven innocent. The defense attorney has to prove beyond a reasonable doubt, within 3 days, that their client could not possibly be guilty, or they're gonna get a GUILTY verdict
In practical terms, this means that if your client is innocent, you have to not only prove that but usually find the actual killer within the three days to show it's a different person. This isn't officially mandated as part of the defense's duties but in pretty much every case it's what Phoenix has to do to exonerate his client, even if he's otherwise proven the killer couldn't be (or is extremely unlikely to be) his client.
Both sides can just show up to the courtroom with new evidence and demand it's accepted as evidence during the trial. There's no verification process for this and no requirement that the other side has access to it pre-trial. You can show up with a letter in hand and halfway through the trial be like "this letter was found in the victim's apartment!" and it becomes part of the case then and there.
There are no restrictions on where or how you can find evidence. You, a defense attorney who doesn't work for the police force and has no equivalent of warrant law, can break into a witness' house and steal evidence from his personal safe to show in court the next day. This is not a crime apparently.
It's normal and accepted that the prosecution will coach all witnesses, usually telling them to lie. It's a huge advantage when you get to interrogate a witness who the prosecution hasn't been able to tell what lies to tell yet. They never face repercussions for this.
The prosecution will frequently falsify evidence. They receive no punishment for this and are allowed to continue practicing law. Witnesses will regularly lie on the stand; they receive no penalty for this and the rest of their testimony is still considered reliable. It's up to the defense attorney to expose every single lie; if you can't prove a word against your client is a lie, even from the mouth of a known liar, then your client must be guilty of it.
All of your trials are overseen by the same judge and he is comically incompetent. This isn't an oversight of the game he is deliberately written to suck at his job, be easily bullied by the prosecution, generally have very little idea what's going on and issue his verdict based on Vibes.
The lawyers will straight up make bets mid-trial with each other like "if you can't find a problem with this next witness' testimony, you have to admit that you're wasting our time and the verdict will be Guilty". The judge lets them do this. This is considered practicing law. Prosecutors will also physically assault other lawyers and the judge in the courtroom but this is okay because it's funny.
The cops work directly for the prosecuting attorney and the prosecuting attorney will openly threaten police witnesses right there on the stand in front of everyone if the witness isn't saying what the prosecution wants them to.
The level of corruption in the prosecutor's office is just. I couldn't describe it in a bullet point. Prosecutors are straightup hitmen for hire and their weapon of choice is the death penalty.
Phoenix gets physically assaulted and robbed by prosecutors and witnesses a lot more than one would reasonably expect. Someone's always there to beat this poor lawyer unconscious and steal evidence from him. He never makes backup copies. That's not the legal system's fault but dude buy a photocopier for your office.
