ArmRender

import com.mojang.blaze3d.systems.RenderSystem;
import net.minecraft.client.Minecraft;
import net.minecraft.client.model.PlayerModel;
import net.minecraft.client.player.AbstractClientPlayer;
import net.minecraft.client.renderer.MultiBufferSource;
import net.minecraft.client.renderer.entity.player.PlayerRenderer;
import net.minecraft.resources.ResourceLocation;
import net.minecraft.world.entity.EquipmentSlot;
import net.minecraft.world.item.Items;
import net.minecraftforge.api.distmarker.Dist;
import net.minecraftforge.client.event.RenderHandEvent;
import net.minecraftforge.eventbus.api.SubscribeEvent;
import net.minecraftforge.fml.common.Mod;
import com.mojang.blaze3d.vertex.PoseStack;
import com.mojang.math.Vector3f;

@Mod.EventBusSubscriber(modid = "displayarmorthingamajin", bus = Mod.EventBusSubscriber.Bus.FORGE, value = Dist.CLIENT)
public class CustomArmRenderer {

    private static final ResourceLocation CUSTOM_ARM_TEXTURE = new ResourceLocation("displayarmorthingamajin", "textures/entity/custom_arm_texture.png");

    @SubscribeEvent
    public static void onRenderHand(RenderHandEvent event) {
        AbstractClientPlayer player = Minecraft.getInstance().player;

        // Check diamond chestplate
        boolean isWearingDiamondChestplate = player.getItemBySlot(EquipmentSlot.CHEST).getItem() == Items.DIAMOND_CHESTPLATE;

        if (isWearingDiamondChestplate) {
            PoseStack poseStack = event.getPoseStack();
            MultiBufferSource bufferSource = event.getMultiBufferSource();
            RenderSystem.setShaderTexture(0, CUSTOM_ARM_TEXTURE);

            // PlayerRenderer and PlayerModel
            PlayerRenderer playerRenderer = (PlayerRenderer) Minecraft.getInstance().getEntityRenderDispatcher().getRenderer(player);
            PlayerModel<AbstractClientPlayer> playerModel = playerRenderer.getModel();

            playerModel.leftArm.visible = true;

            playerModel.setupAnim(player, 0, 0, 0, 0, 0);

            poseStack.pushPose();

            poseStack.translate(0.0D, -0.5D, -1.0D);
            poseStack.mulPose(Vector3f.XP.rotationDegrees(playerModel.leftArm.xRot));
            poseStack.mulPose(Vector3f.YP.rotationDegrees(playerModel.leftArm.yRot));
            poseStack.mulPose(Vector3f.ZP.rotationDegrees(playerModel.leftArm.zRot));


            // Get swing
            float swingProgress = player.getAttackAnim(0.0F);
            float swingAnimation = swingProgress;
            poseStack.mulPose(com.mojang.math.Vector3f.YP.rotation(swingAnimation));
            poseStack.mulPose(com.mojang.math.Vector3f.ZP.rotation(swingAnimation));
            poseStack.mulPose(com.mojang.math.Vector3f.XP.rotation(playerModel.leftArm.xRot));
            poseStack.mulPose(com.mojang.math.Vector3f.YP.rotation(playerModel.leftArm.yRot));
            poseStack.mulPose(com.mojang.math.Vector3f.ZP.rotation(playerModel.leftArm.zRot));

            // Texture
            playerModel.leftArm.render(poseStack, bufferSource.getBuffer(playerModel.renderType(CUSTOM_ARM_TEXTURE)),
                    event.getPackedLight(), 0, 1.0F, 1.0F, 1.0F, 1.0F);

            poseStack.popPose();
        }
    }
}