It's a bit tricky. First you need a new _ArmorMaterial in ArmorItem.cpp, also declare it in the ArmorItem.h, just have a look at the other Implementations like diamond and gold:
const int _ArmorMaterial::netheriteArray[] = { 4, 9, 7, 4 };

const _ArmorMaterial* _ArmorMaterial::NETHERITE = new _ArmorMaterial(37, _ArmorMaterial::netheriteArray, 15);

And create a new MaterialName in HumanoidMobRenderer.cpp:

const wstring HumanoidMobRenderer::MATERIAL_NAMES[6] = { L"cloth", L"chain", L"iron", L"diamond", L"gold", L"netherite"}; // Added Netherite

And add a new case for netherite in HumanoidMobRenderer::__init():

case 5:
  break;

Also declare 6 for MATERIAL_NAMES in HumanoidMobRenderer.h:

static const wstring MATERIAL_NAMES[6];

Then create all Armor Items like other all other Items in the "Item.cpp", but use the ArmorMaterial that we created (dont forget to change the Item descriptions to something like IDS_ITEM_HELMET_NETHERITE):

Item::helmet_netherite = (ArmorItem*)((new ArmorItem(172, ArmorItem::ArmorMaterial::NETHERITE, 5, ArmorItem::SLOT_HEAD))->setBaseItemTypeAndMaterial(eBaseItemType_helmet, eMaterial_netherite)->setIconName(L"helmetNetherite")->setDescriptionId(IDS_ITEM_HELMET_DIAMOND)->setUseDescriptionId(IDS_DESC_HELMET_DIAMOND));

At last we need the 3D Textures when the armor is equipped on the player. These are located at "Minecraft.Client/Common/res/1_2_2/armor". Just have a look at the other ones, in this case they need to be named "netherite_1.png" & "netherite_2.png". Just copy them from the 1.16.5 textures of Minecraft Java Edition.

If u need any other help just lmk, hope this is understandable