TLDR; namespace->data->zombiesattackeverything->tags->entity_types->zombie_friendly_type.json

FOR THE 1.21.1 VERSION, RENAME "entity_types" FOLDER TO "entity_type". SO REMOVE THE S AT THE END.

First make a datapack, pretty simple, just make a folder named whatever you want, then in that folder add a file named "pack.mcmeta" (without the qoutes obviously) and add this text in it:

{
  "pack": {
    "description": "this datapack does yadayada cool description whatever",
    "pack_format": 61
  }
}

now the description is just what the text under the datapack is gonna be when you add it to your world, it doesnt really matter that much, the pack_format doesnt really matter either, its there to show what version its for but it being 1 (for example) doesnt stop the datapack from functioning on the latest version.

then add another folder called "data" and in there add a folder called "zombiesattackanything", in that add a folder called "entity_types" and in that add a file called "zombie_friendly_type.json". in there add the following text:

{
  "replace": false,
  "values": [
    "namespace:entityid",
    "minecraft:zombie"
  ]
}

 and voilà, done!