Where to "post" or place the file (e.g., the /plugins or /mods folder).

If you absolutely must run a legacy Java applet, you have two options:

While the browser plugin is dead, the need to run Java applications locally remains high. Users searching to "download Java" are typically looking to install the JRE or the Java Development Kit (JDK).

Downloading the plugin is not enough; the environment must be configured.