.NET Framework, .NET Core 3+ and .NET 5+ can expose a COM object to the COM world. This world includes VB6 and everything that can do COM.

a) Create a .NET 5 "NetClassLibrary" Class Library project (using Visual Studio for example)

b) Add a NetComObject.cs file with the following code for example:

using System;
using System.Runtime.InteropServices;

namespace NetClassLibrary
{
    [ComVisible(true)]
    [Guid("cfb2ee8b-f2ec-447c-9f0d-ac19c9a6cc26")] // TODO: change this GUID
    public class NetComObject
    {
        public string SayHello() => "Hello World from .NET " + RuntimeInformation.FrameworkDescription;
    }
}

c) Modify the .csproj to set the EnableComHosting property to true and make sure you compile for x86 as VB6 is a 32-bit program (note if you use Excel's 64-bit VBA, not VB6, you must compile for 64-bit), like this for example:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
    <EnableComHosting>true</EnableComHosting>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
    <PlatformTarget>x86</PlatformTarget>
  </PropertyGroup>

</Project>

d) Compile, start an admin prompt and run this:

regsvr32 yourPathTo\NetClassLibrary.comhost.dll

e) Create a VB6 project, add this VB6 code:

Private Sub Form_Load()

    Dim obj As Object
    Set obj = CreateObject("new:{cfb2ee8b-f2ec-447c-9f0d-ac19c9a6cc26}") // TODO: use the same GUID!

    MsgBox obj.SayHello()

End Sub

f) Run

Note 1: you can use a ProgId attribute too, like COM object often do, you don't have to stick to Guids only...

Note 2: I've only used IDispatch ("Lazy Loading") interface for this demonstration but you can use IUnknown-derived interfaces ("Early Loading") and TLBs too. Unfortunately .NET 5 doesn't create the .TLB for you, you'll have to build it by yourself or create it with another tool and import it in .NET and/or in VB6.