Yes, Java SE is 64-bit
Java Web Start is unfortunately deprecated since Java 9 and has been removed in Java 11
Processing brings the Java 1.8.0 Update 202 JVM (→ Java 8) out of the box (At least on my Windows PC)
As I don’t use Apple products I don’t know what they are trying to do but Java is probably going to stay being available for MacOS.
Java SE 15.0 (→ Java 5) was released in 2004 and got (public) security updates until 2009. I wouldn’t advise you using Software needing a version of Java that old.