Many to many relationship using ASP.NET with HTML, CSS and Javascript

Question

I need your help. I am developing a web application with C# ASP.NET and I have a problem. I am using the N-layer architecture and I am using services. So, I don't know how to implement the many-to-many relationship in the create method.

Interface Class

public interface IInformacionBasicaEstudianteService
{
    Task<List<InformacionBasicaEstudiante>> Lista();

    Task<InformacionBasicaEstudiante> ObtenerInformacionBasicaEstudiante(int pIdUsuario);

    Task<InformacionBasicaEstudiante> Crear(InformacionBasicaEstudiante modelo);

    Task<InformacionBasicaEstudiante> Editar(InformacionBasicaEstudiante modelo);
}

Method within the service class

public async Task<InformacionBasicaEstudiante> Crear(InformacionBasicaEstudiante entidad)
{
    try
    {
        InformacionBasicaEstudiante infoCreada = await _repository.Crear(entidad);
        if (infoCreada.IdInfoBasEstudiante == 0)
            throw new TaskCanceledException("NO SE PUDO GUARDAR LA INFORMACIÓN");

        IQueryable<InformacionBasicaEstudiante> query = await _repository.Consultar(u => u.IdInfoBasEstudiante == infoCreada.IdInfoBasEstudiante);
        infoCreada = query.Include(r => r.IdDptoExpDocNavigation).Include(r => r.IdDptoNacNavigation)
                .Include(r => r.IdDptoResidenciaNavigation).Include(r => r.IdEpsNavigation)
                .Include(r => r.IdEstratoNavigation).Include(r => r.IdEtniaNavigation)
                .Include(r => r.IdFrecCambioDomicNavigation).Include(r => r.IdGeneroNavigation)
                .Include(r => r.IdInfoBasAcudienteNavigation).Include(r => r.IdMpioExpDocNavigation)
                .Include(r => r.IdMpioNacNavigation).Include(r => r.IdMpioResidenciaNavigation)
                .Include(r => r.IdPaisOrigenNavigation).Include(r => r.IdParentescoNavigation)
                .Include(r => r.IdPobVictConfNavigation).Include(r => r.IdPropiedadViviendaNavigation)
                .Include(r => r.IdResguardoNavigation).Include(r => r.IdSisbenNavigation)
                .Include(r => r.IdTipoSangreNavigation).Include(r => r.IdTipoViviendaNavigation)
                .Include(r => r.IdUsuarioNavigation).Include(r => r.IdZonaResidenciaNavigation)
                .First();

        return infoCreada;
    }
    catch (Exception ex)
    {
        throw;
    }
}

ER-Diagram enter image description here

I need to know how to implement those relationships within the create service method so that when I create a register, it is directly saved in the many-to-many table.

Thank you very much.