DIPlugins.cs

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Options;

var services = new ServiceCollection();
services.AddSingleton<RootService>();
services.AddSingleton<IService, RootService>();
PluginImplementation.Configure(services);
using var rootSp = services.BuildServiceProvider();

var pluginRegistration = rootSp.GetRequiredService<IOptionsMonitor<PluginRegistration>>()
    .Get(PluginImplementation.Name);

var pluginServices = new ServiceCollection();
using var scope = rootSp.CreateScope();
pluginRegistration.Configure(scope.ServiceProvider, pluginServices);
using var pluginSp = pluginServices.BuildServiceProvider();

var fromRoot = pluginSp.GetRequiredService<RootService>();
Console.WriteLine(fromRoot); // RootService
var fromPlugin = pluginSp.GetRequiredService<IService>();
Console.WriteLine(fromPlugin); //PluginOverride

interface IService { }

class RootService : IService { }

class PluginOverride : IService { }

interface IPlugin { }

class PluginImplementation : IPlugin
{
    public const string Name = nameof(PluginImplementation);
    public static void Configure(ServiceCollection rootServices)
    {
        rootServices.Configure<PluginRegistration>(Name, o => o.Configure = static (sp, services) =>
        {
            services.AddSingleton<IService, PluginOverride>();
            services.AddSingleton(_ => sp.GetRequiredService<RootService>());
        });
    }
}

class PluginRegistration
{
    public Action<IServiceProvider, ServiceCollection> Configure = delegate { };
}