Cross platform is enabled by default in the PlayStation 5 version of Honkai: Star Rail. If you would rather disable this feature so that the game will only connect with other PS5 users, then you can check out this tutorial from Seeking Tech to see how you can properly do so.
1. To start off, you need to make your way to the Settings menu in Honkai: Star Rail.
You can do so from the title screen, by pressing L3 and then selecting the gear icon. You can also select the same gear icon from the phone menu.
2. In Settings, use the L1 and R1 buttons of the DualSense to change tabs until you get to the Account Settings tab.
3. From there, select the “Only play with users from PlayStation Network” setting.
4. By default, this setting is set to Off, which means that you are able to connect with other users that are playing on the PC, Android or iOS platforms.
To disable cross platform connections, make sure you set the “Only play with users from PlayStation Network” setting to On.
5. You can then exit the Settings menu.
When using the multiplayer features in Honkai: Star Rail, your connection will now be limited to other PS5 users.