Like its predecessor, Persona 4 Golden, Persona 5 Royal also has multiple endings. You can get the conclusion you want to be based on your choices at some parts of the game, as well as some requirements, especially your Confidant level with a specific arcana.
According to RPG Site, Persona 5 Royal has six known endings with three bad ones, a normal one, and then true endings (one with additional scenes). Both the normal and one of the bad ends were already available on the base game, but the rest are new with P5R.
Without further ado, here's your guide on how to get the ending you wish to witness.
How to Get the True Ending
The "true" ending of Persona 5 Royal is deemed as the canonical conclusion to Joker and his crew, so it's also regarded as the best ending, plus there are also several bonus scenes you can get if you meet the right requirements.
However, any player who wants to get the true ending should access the game's third semester, which players could miss entirely if they make a few bad choices. To ensure you unlock this semester, you should ensure Maruki's Confidant cooperation to level nine before Nov. 17.
It would be best to prioritize Maruki's Confidant, especially as November creeps in.
Besides Maruki's Confidant (Councillor), you'll also want to level up Kasumi Yoshizawa's Confidant (Faith) by Dec. 18.
Plus, you'll also want to reach the maximum level of Goro Akechi's Confidant cooperation (Justice), and you should reach it by Nov. 17 if you want to get the true ending with the bonus scenes.
Now, here's the final requirement for the best and true ending: choose the right dialogues on Nov. 20.
Basically, you have to refuse to cooperate with Sae Niijima and protect your friends at all costs. Deny her and deny all knowledge of your friends and accomplices ("No." and "Never heard of them.").
Lastly, choose "There were no such people."
Goro Akechi'll also questions you, but none of the answers will actually matter as any answer you make will progress to the true ending. By Jan. 9 and Feb. 2, there will be choices that you need to make, but the answers should be obvious to reach a good conclusion.
Bad Ending #1
This is perhaps the easiest Persona 5 Royal ending to obtain by simply ignoring the treasure inside the Palace as well as the Palace's deadline. Skip this opportunity, and you'll end up getting the first Bad ending.
Bad Ending #2
The second bad ending will depend on your choices when you're being interrogated by Sae Niijima and reveal who your Phantom Thieves accomplices are.
Bad Ending #3
The last bad ending will be based on your decision on dates 1/9 and 2/2. Simply accept the offers that have been made to you. This should prompt the third bad ending scene.
The normal ending is the common ending you'll get in the base game, meaning you hold firm with the interrogation, but you fail to continue to the third semester.