Cannot reference my nuget package namespace - nuget-package

Question

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?