The netcoreapp3.1 was the issue that nopcommerce couldnt find the plugin and show it in the local plugins list. To fix it i added this in the csproj file fo the plugin project:
<PropertyGroup>
<TargetFramework>netcoreapp3.1</TargetFramework>
<AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
</PropertyGroup>
Is this a common issue or did this only occur for me? If it occurs for others then here is the solution, and maybe this fix should be included in the tutorial.