Where is the path for the java command defined in Ubuntu?

I am struggling to get my Ubuntu 20.04 to get to recognize the correct java version. I have set the correct environmental variable inside a shell script in /etc/profile.d/

My $JAVA_HOME output:

root@Ubuntu-2004-focal-64-minimal /etc/profile.d # echo $JAVA_HOME
/home/miw/java/jdk-15.0.2

.. this points to the correct java version i want to use! (15.0.2)

Furthermore i appended this home directory to my $PATH variable:

root@Ubuntu-2004-focal-64-minimal /etc/profile.d # echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/home/miw/java/jdk-15.0.2

My Maven correctly recognizes the version, since it uses the $JAVA_HOME variable to determine the version.

root@Ubuntu-2004-focal-64-minimal / # mvn -version
Apache Maven 3.6.3
Maven home: /usr/share/maven
Java version: 15.0.2, vendor: Oracle Corporation, runtime: /home/miw/java/jdk-15.0.2

Now: My question is where inside the startup process or generally in which file the default java command directory is defined. I DO NOT WANT just a solution to my problem. (like some automated programs where i can switch between alternatives etc.)

The system somehow still recognizes the preinstalled java version located inside /usr/lib/jvm/java-11-openjdk-amd64/bin as its default... and indeed if i take a look inside the /usr/bin directory there is a symlink called java which points to the /etc/alternatives/java symlink that points to /usr/lib/jvm/java-11-openjdk-amd64/bin.

My next guess was that this is due to the $PATH hierarchy since /usr/bin is defined BEFORE my custom $JAVA_HOME.

Well.. i tried deleting those symlinks both, hoping that the system would finally reach my custom $JAVA_HOME directory in search of a java executable.... what happes now is that when i try to show the java version an error occurs:

root@Ubuntu-2004-focal-64-minimal /etc/alternatives # java -version
-bash: /usr/bin/java: No such file or directory

Well of course its not there anymore because i have deleted it... Why does the system take the /usr/bin/java for granted anyway ? Where is this exact path for the java command defined ?

I verified this by creating my own symlink inside /usr/bin that points to my custom directory.. and tada..

root@Ubuntu-2004-focal-64-minimal /etc/alternatives # ln -s /home/miw/java/jdk-15.0.2/bin/java /usr/bin/java
root@Ubuntu-2004-focal-64-minimal /etc/alternatives # java -version
openjdk version "15.0.2" 2021-01-19
OpenJDK Runtime Environment (build 15.0.2+7-27)
OpenJDK 64-Bit Server VM (build 15.0.2+7-27, mixed mode, sharing)

So can anybody explain to me what actually happened here ? Why is the system assuming that /usr/bin/java MUST be where the java command is found ? This somehow destroys the purpose of the $PATH variable in the first place ! What am i not getting here ?