Example of a Multi-targeted .NET Standard Library for Xamarin. This library targets .NET Standard 2.0, Xamarin.iOS and Xamarin.Android, it is here as a reference

ControlsLibrary.csproj

<Project Sdk="MSBuild.Sdk.Extras/2.0.24">

  <PropertyGroup Label="Globals">
    <SccProjectName>SAK</SccProjectName>
    <SccProvider>SAK</SccProvider>
    <SccAuxPath>SAK</SccAuxPath>
    <SccLocalPath>SAK</SccLocalPath>
  </PropertyGroup>

  <PropertyGroup>
    <!--Don't auto add files for me, I will tell you how to do it -->
    <EnableDefaultCompileItems>false</EnableDefaultCompileItems>

    <!--The frameworks we're targeting-->
    <TargetFrameworks>MonoAndroid90;netstandard2.0;Xamarin.iOS10;</TargetFrameworks>

    <!--Assembly and Namespace info -->
    <RootNamespace>Company.Example.Controls</RootNamespace>
    <Product>$(AssemblyName) ($(TargetFramework))</Product>
    <AssemblyVersion>1.0.0.0</AssemblyVersion>
    <AssemblyFileVersion>1.0.0.0</AssemblyFileVersion>
    <Version>1.0.0.0</Version>
    <NeutralLanguage>en</NeutralLanguage>

    <!-- Helper to add defines-->
    <DefineConstants>$(DefineConstants);</DefineConstants>

    <!--Version of C# to use -->
    <LangVersion>default</LangVersion>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>

  </PropertyGroup>

  <!-- Define what happens on build and release -->
  <PropertyGroup Condition=" '$(Configuration)'=='Debug' ">
    <DebugType>full</DebugType>
    <DebugSymbols>true</DebugSymbols>
    <GenerateDocumentationFile>false</GenerateDocumentationFile>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)'=='Release' ">
    <DebugType>pdbonly</DebugType>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
    
    <!-- If we want a nuget package on build, add this -->
    <!-- <GeneratePackageOnBuild>true</GeneratePackageOnBuild> -->
  </PropertyGroup>


  <ItemGroup>

    <!-- This None is here so the conditionally included files show up in the Solution Explorer -->
    <None Include="**\*.cs;**\*.xml;**\*.axml;**\*.png" Exclude="obj\**\*.*;bin\**\*.*;bin;obj" />

    <!--NuGet packages for all platforms-->
    <PackageReference Include="SkiaSharp" Version="1.68.0" />
    <PackageReference Include="SkiaSharp.Views.Forms" Version="1.68.0" />
    <PackageReference Include="Xamarin.Forms" Version="3.6.0.293080" />

    <!--Project references for all platforms-->
    <!-- <ProjectReference Include="..\..\..\Framework\Company.SomeLibrary.csproj" /> -->

    <!--Update all .xaml files in shared -->
    <EmbeddedResource Update="Shared\**\*.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>

    <!--Compile all .xaml files in shared-->
    <EmbeddedResource Update="Shared\**\*.xaml">
      <Generator>MSBuild:Compile</Generator>
    </EmbeddedResource>

    <!--Compile all .cs files in shared-->
    <Compile Include="Shared\**\*.cs" />
  </ItemGroup>

  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
  </ItemGroup>

  <!--Do this exclusively for Android-->
  <ItemGroup Condition=" '$(TargetFramework)' == 'MonoAndroid90' ">
    <Compile Include="Platforms\Android\**\*.cs" />
    
    <!-- We can reference assemblies... -->
    <Reference Include="System.Data" />
    
    <!-- We can also reference NuGet packages... -->
    <PackageReference Include="FAB.XamarinAndroid" Version="1.6.4" />

    <!-- If we have resources on Android, add these... -->
    <!--
    <AndroidResource Include="Resources\**\*.axml" />
    <AndroidResource Include="Resources\**\*.xml" />
    <AndroidResource Include="Resources\**\*.png" />
    <Compile Include="Resources\Resource.designer.cs" />
    -->
  </ItemGroup>

  <PropertyGroup Condition=" '$(TargetFramework)' == 'MonoAndroid90' ">
    <AndroidResgenFile>Resources\Resource.designer.cs</AndroidResgenFile>
  </PropertyGroup>

  <!--Do this exclusively for iOS-->
  <ItemGroup Condition=" '$(TargetFramework)' == 'Xamarin.iOS10' ">
    <Reference Include="System.Data" />
    <Compile Include="Platforms\iOS\**\*.cs" />
    <!--<Copy SourceFiles="$(NuSpecPath)" DestinationFolder="$(PackageSource)\Resources"/>-->

    <!--<Content Include="Resources\*.png">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </Content>-->
  </ItemGroup>
  <ItemGroup>
    <Folder Include="Platforms\Android\" />
    <Folder Include="Platforms\iOS\" />
    <Folder Include="Platforms\" />
    <Folder Include="Shared\" />
    <Folder Include="Resources\" />
  </ItemGroup>

</Project>