Multi Target for Xamarin, you need to add the global.json file too

global.json

{
  "sdk": {
    "version": "3.1.101"
  },
  "msbuild-sdks": {
    "MSBuild.Sdk.Extras": "2.0.54"
  }
}

MultiTarget.csproj

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

  <PropertyGroup>
    <TargetFrameworks>netstandard2.1;netcoreapp3.0;MonoAndroid90;MonoAndroid10.0;Xamarin.iOS10</TargetFrameworks>
    <Product>$(AssemblyName) ($(TargetFramework))</Product>
    <AssemblyVersion>1.0.0.0</AssemblyVersion>
    <AssemblyFileVersion>1.0.0.0</AssemblyFileVersion>
    <Version>1.0.0.0</Version>
    <PackageVersion>1.0.0.0</PackageVersion>
    <PackOnBuild>true</PackOnBuild>
    <NeutralLanguage>en</NeutralLanguage>
    <LangVersion>default</LangVersion>
    <DefineConstants>$(DefineConstants);</DefineConstants>
    <UseFullSemVerForNuGet>false</UseFullSemVerForNuGet>
    <EnableDefaultCompileItems>false</EnableDefaultCompileItems>
    <LangVersion>latest</LangVersion>
    <Configurations>Debug;Release;Test</Configurations>
  </PropertyGroup>

  <PropertyGroup Condition=" '$(Configuration)'=='Debug' ">
    <DebugType>full</DebugType>
    <DebugSymbols>true</DebugSymbols>
    <GenerateDocumentationFile>false</GenerateDocumentationFile>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)'=='Release' ">
    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
    <DebugType>pdbonly</DebugType>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
  </PropertyGroup>
  <PropertyGroup Condition="'$(Configuration)'=='Test'">
    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
    <DebugType>pdbonly</DebugType>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="**\*.shared.cs" />
    <Compile Include="**\*.shared.xaml.cs" />
  </ItemGroup>

  <ItemGroup Condition=" $(TargetFramework.StartsWith('netstandard')) ">
  </ItemGroup>

  <ItemGroup Condition=" $(TargetFramework.StartsWith('MonoAndroid')) ">
    <Compile Include="**\*.android.cs" />
  </ItemGroup>

  <ItemGroup Condition=" $(TargetFramework.StartsWith('Xamarin.iOS')) ">
    <Compile Include="**\*.apple.cs" />
  </ItemGroup>
</Project>