Entity Framework Core - One-to-one relationships not creating foreign key

I guess that you have a small misunderstanding about one-to-one relationships (or I have misunderstood your question). Basically it is covered in the EF Core docs - One-to-one relationships:

A required relationship ensures that every dependent entity must be associated with some principal entity. However, a principal entity can always exist without any dependent entity. That is, a required relationship does not indicate that there will always be a dependent entity. There is no way in the EF model, and also no standard way in a relational database, to ensure that a principal is associated with a dependent. If this is needed, then it must be implemented in application (business) logic. See Required navigations for more information.

If you want to simulate the "truly" one-to-one relationship the only option via EF is to use owned entity types as mentioned in the linked previously "Required navigations" portion of the docs:

There is one exception to this rule - when the principal and dependent types are sharing the same table in a relational database, or contained in a document. This can happen with owned types, or non-owned types sharing the same table. In this case, the navigation property from the principal to the dependent can be marked as required, indicating that the dependent must exist.

If you have several one-to-one relationships between tables (i.e. Soggetto can have a depended Azienda and Azienda can have another depended Soggetto) then you will need to have several properties. For example something along these lines (not tested):

public class Soggetto 
{
    public long Id { get; set; }

    public virtual Azienda ChildAzienda { get; set; }

    public long? ParentAziendaId { get; set; }
    public virtual Azienda? ParentAzienda { get; set; }
}

public class Azienda 
{
    public long Id { get; set; }

    public long ParentSoggettoId { get; set; }
    public virtual Soggetto ParentSoggetto { get; set; }

    public virtual Soggetto ChildSoggetto { get; set; }
}

And map them correspondingly with the Fluent API. For example:

public class SoggettoConfiguration : IEntityTypeConfiguration<Soggetto>
{
    public void Configure(EntityTypeBuilder<Soggetto> builder)
    {
        builder
            .HasOne(soggetto => soggetto.ChildAzienda)
            .WithOne(azienda => azienda.ParentSoggetto)
            ;
    }      
}

public class AziendaConfiguration : IEntityTypeConfiguration<Azienda>
{
    public  void Configure(EntityTypeBuilder<Azienda> builder)
    {
        builder
            .HasOne(azienda => azienda.ChildSoggetto)
            .WithOne(soggetto => soggetto.ParentAzienda)
            ;
    }
}

First, a clear explanation of the relationship is needed.

In your current model, an Azienda is required to create a Soggetto (since the id is not nullable).

Conversely, a Soggetto is required to create an Azienda.

This is a contradiction. Neither can be created first.

Edited

Given the clarification provided by the questioner, it appears that:

• Azienda represents a Tenant.
• Every Soggetto should be linked to an Azienda(Tenant) to denote ownership.

This translates to the following in terms of Entity Framework (EF) modeling:

If an Azienda can have multiple Soggettos:

public class Azienda
{
    public long Id { get; set; }
    public List<Soggetto> Soggettos { get; set; }
}
public class Soggetto
{
    public long Id { get; set; }

    public virtual Azienda Azienda { get; set; }
    public long AziendaId { get; set; }

    internal class Configuration : IEntityTypeConfiguration<Soggetto>
    {
        public void Configure(EntityTypeBuilder<Soggetto> builder)
        {
            builder
                .HasOne(o => o.Azienda)
                .WithMany(o => o.Soggettos);
        }
    }
}

Generated migration code:

migrationBuilder.CreateTable(
    name: "Azienda",
    columns: table => new
    {
        Id = table.Column<long>(type: "bigint", nullable: false)
            .Annotation("SqlServer:Identity", "1, 1")
    },
    constraints: table =>
    {
        table.PrimaryKey("PK_Azienda", x => x.Id);
    });

migrationBuilder.CreateTable(
    name: "Soggetto",
    columns: table => new
    {
        Id = table.Column<long>(type: "bigint", nullable: false)
            .Annotation("SqlServer:Identity", "1, 1"),
        AziendaId = table.Column<long>(type: "bigint", nullable: false)
    },
    constraints: table =>
    {
        table.PrimaryKey("PK_Soggetto", x => x.Id);
        table.ForeignKey(
            name: "FK_Soggetto_Azienda_AziendaId", 
            column: x => x.AziendaId,
            principalTable: "Azienda",
            principalColumn: "Id",
            onDelete: ReferentialAction.Cascade);
    });
migrationBuilder.CreateIndex(
    name: "IX_Soggetto_AziendaId",
    table: "Soggetto",
    column: "AziendaId");

If an Azienda can have only one Soggetto:

public class Azienda
{
    public long Id { get; set; }
    public Soggetto Soggetto { get; set; }
}
public class Soggetto
{
    public long Id { get; set; }

    public virtual Azienda Azienda { get; set; }
    public long AziendaId { get; set; }

    internal class Configuration : IEntityTypeConfiguration<Soggetto>
    {
        public void Configure(EntityTypeBuilder<Soggetto> builder)
        {
            builder
                .HasOne(o => o.Azienda)
                .WithOne(o => o.Soggetto);
        }
    }
}

Genereted migration code:

migrationBuilder.CreateTable(
    name: "Azienda",
    columns: table => new
    {
        Id = table.Column<long>(type: "bigint", nullable: false)
            .Annotation("SqlServer:Identity", "1, 1")
    },
    constraints: table =>
    {
        table.PrimaryKey("PK_Azienda", x => x.Id);
    });

migrationBuilder.CreateTable(
    name: "Soggetto",
    columns: table => new
    {
        Id = table.Column<long>(type: "bigint", nullable: false)
            .Annotation("SqlServer:Identity", "1, 1"),
        AziendaId = table.Column<long>(type: "bigint", nullable: false)
    },
    constraints: table =>
    {
        table.PrimaryKey("PK_Soggetto", x => x.Id);
        table.ForeignKey(
            name: "FK_Soggetto_Azienda_AziendaId",
            column: x => x.AziendaId,
            principalTable: "Azienda",
            principalColumn: "Id",
            onDelete: ReferentialAction.Cascade);
    });

migrationBuilder.CreateIndex(
    name: "IX_Soggetto_AziendaId",
    table: "Soggetto",
    column: "AziendaId",
    unique: true); // <- set foreign key unique.

You can simply create Soggeto like this:

var newItem = new Soggetto
{
    Azienda = new Azienda()
};
context.Soggetto.Add(newItem);
await context.SaveChangesAsync();

Or conversely (in the case of a 1:1 relationship):

var newItem = new Azienda
{
    Soggetto = new Soggetto()
};
context.Azienda.Add(newItem);
await context.SaveChangesAsync();