In some cases i can see that the UI of the Player is loading and visible before the Player is in the Server with the Player, is that right like that?
I am not sure what you mean by you can see it visible before the player is in the server, but I don’t think UI loads before they have joined the server. Sometimes if you try to call a UI element in a script too early the script will run faster than the UI element has time to load in.
Yes, just put that on replicated first
When a player join you can clone from replicated first the ui on PlayerGui
Read more here: ReplicatedFirst | Roblox Creator Documentation
I mean that when i put something in the ReplicatedFirst, i can see my UI first before the Player is in the server
I have it in ReplicatedFirst, so is that right that its showing before the Player is in the Server?
Usually Yes, Gui loads before the player, as when the player joins the game, the UI is cloned and added into their PlayerGui folder which therefore, shows up on their screen. Thats why its called StarterGui
Yes you can clone ui when player join with a local script from replicated first to startergui
Il you are using a loading ui remember to use ReplicatedFirst | Roblox Creator Documentation ( RemoveDefaultLoadingScreen)