Java

Which Java your game runs on, the experimental newer-Java switch, and when to touch the Java path setting.

Performance
Settings, Sour Client tab. The Java path is under Java runtime ; the per-profile Run on Java switch is on the Profiles page.
Settings, Sour Client tab. The Java path is under Java runtime; the per-profile Run on Java switch is on the Profiles page.

What it runs on

1.21.11 runs on the modern Java the launcher brings with it. You never install Java yourself.

Run on Java 25

On a Fabric profile, the Profiles page has Run on Java 25. It is experimental and can give slower processors a few more frames. Java 25 downloads the first time you play with it on. If a mod you added does not like it, switch it off again.

The Java path setting

Settings, Sour Client, Java runtime has a Java path. Leave it on the default (java): the launcher then picks the right bundled runtime for each version. Only change it if support asks you to. Reset puts it back.