Patching / Overriding is poorly explained in the jsonnet documentation. Here's the tl;dr:

main.jsonnet

// See https://jsonnet.xrt0x.com/j/VigL64fxlx4
//
// TL;DR:
// * `:`   -> Set a field, preserving its visibility. If this is the root / base object, the field is public. 
// * `::`  -> Set a field and also make it private / hidden.
// * `:::` -> Set a field and also make it public / visible.
//
// Prefix any of these with a `+` (so `+:` / `+::` / `+:::`) to patch a field / object instead. What "patch"ing
// means depends on the type of the field. Objects are merged, lists are appended to, and strings are concatenated.

{
  a: {
    pub1: "pub1",
    pub2: "pub2",
    pub3: "pub3",
        
    priv1:: "priv1",
    priv2:: "priv2",
    priv3:: "priv3",
        
    _priv1: self.priv1,
    _priv2: self.priv2,
    _priv3: self.priv3,
  }
} + {
  a+: {
    // : sets a field, preserving its visiblity
    pub1: "override1",
    priv1: "overridepriv1"
  }
} + {
  a+: {
    // :: sets a field, overriding its visiblity to private
    pub2:: "override2",
    priv2:: "overridepriv2"
  }
} + {
  a+: {
    // ::: sets a field, overriding its visibility to public
    pub3::: "override3",
    priv3::: "overridepriv3"
  }
}

output.json

{
   "a": {
      "_priv1": "overridepriv1",
      "_priv2": "overridepriv2",
      "_priv3": "overridepriv3",
      "priv3": "overridepriv3",
      "pub1": "override1",
      "pub3": "override3"
   }
}