Asp.net core 2.1 - How to serve multiple angular apps?

Well, I finally solved it and it works like this:

public void ConfigureServices(IServiceCollection services)
{
    ...

    services.AddSpaStaticFiles();

    ...
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    ...

    app.UseSpaStaticFiles(new StaticFileOptions
    {
        FileProvider = new PhysicalFileProvider(Path.Combine(Directory.GetCurrentDirectory(), "wwwroot"))
    });

    app.UseMvc();

    app.Map("/app", client =>
    {
        client.UseSpa(spa =>
        {
            spa.Options.SourcePath = "wwwroot/app";
            spa.Options.DefaultPageStaticFileOptions = new StaticFileOptions
            {
                FileProvider = new PhysicalFileProvider(Path.Combine(Directory.GetCurrentDirectory(), "wwwroot/app"))
            };
        });
    }).Map("/admin", admin =>
    {
        admin.UseSpa(spa =>
        {
            spa.Options.SourcePath = "wwwroot/admin";
            spa.Options.DefaultPageStaticFileOptions = new StaticFileOptions
            {
                FileProvider = new PhysicalFileProvider(Path.Combine(Directory.GetCurrentDirectory(), "wwwroot/admin"))
            };
        });
    });
}

Also, don't forget to go into the index.html file of each app and set the base tag accordingly like this:

//for the client application:
<base href="/app/">

//for the admin application
<base href="/admin/">