Entity Spawn Packets

Non-living (and other custom) entities need a spawn packet so the client knows to create them. Architectury provides a ready-made one through NetworkManager.

Using the Architectury spawn packet

Override your entity's getAddEntityPacket to return NetworkManager.createAddEntityPacket:

public class MyProjectile extends Entity {
    // ...

    @Override
    public Packet<ClientGamePacketListener> getAddEntityPacket(ServerEntity serverEntity) {
        return NetworkManager.createAddEntityPacket(this, serverEntity);
    }
}

That's all most entities need - the packet carries the entity type, id, position, rotation, and velocity.

Sending extra spawn data

If your entity needs additional data available the moment it spawns on the client, implement dev.architectury.extensions.network.EntitySpawnExtension:

public class MyProjectile extends Entity implements EntitySpawnExtension {
    private int variant;

    @Override
    public void saveAdditionalSpawnData(FriendlyByteBuf buf) {
        buf.writeVarInt(variant);
    }

    @Override
    public void loadAdditionalSpawnData(FriendlyByteBuf buf) {
        this.variant = buf.readVarInt();
    }

    @Override
    public Packet<ClientGamePacketListener> getAddEntityPacket(ServerEntity serverEntity) {
        return NetworkManager.createAddEntityPacket(this, serverEntity);
    }
}

saveAdditionalSpawnData runs on the server when the spawn packet is built; loadAdditionalSpawnData runs on the client when it's received. Read the fields back in the same order you wrote them.