Cannot reference my nuget package namespace

Repro

I have created my FIRST nuget package. Package project targets net standard 2.0. It is built and published by azure pipeline:

...
- task: VSBuild@1
  ...

- task: VSTest@2
  ...

- task: NuGetCommand@2
  inputs:
    command: pack
    packDestination: '$(Build.ArtifactStagingDirectory)/Nuget/MyPackage'
    packagesToPack: '**/MyPackage.EventBus.nuspec'

- task: NuGetCommand@2
  displayName: 'NuGet push'
  inputs:
    command: push
    packagesToPush: '$(Build.ArtifactStagingDirectory)/Nuget/MyPackage/*.nupkg'
    publishVstsFeed: 'MyFeed'
    allowPackageConflicts: true

I have a nuspec file which has defined id, version, title, owner, license etc. and of course files:

<file src="MyPackage.EventBus\Factory\**" target="content\Factory" />
<file src="MyPackage.EventBus\Interfaces\**" target="content\Interfaces" />
<file src="MyPackage.EventBus\Null\**" target="content\Null" />
<file src="MyPackage.EventBus\ServiceBus\**" target="content\ServiceBus" />

<file src="MyPackage.EventBus\*.cs" target="content" />
<file src="MyPackage.EventBus\MyPackage.EventBus.csproj" target="content" />

Next I have installed my package in azure function v3 project with net core 3.1.

Problem

After installation I cannot reference any class or namespace from my package e.g.:

namespace MyPackage.EventBus
{
    public abstract class BaseEvent

Tried

I have checked my nuget package file in NuGet Package Explorer:

It seems that there is no dll file. Should it be there in package or not? What to do to contain dll file in package if it is neccessary?