FilledForm.eto.cs

partial class FilledForm : Form
{
    void InitializeComponent()
    {
        Title = "Test Form";
        ClientSize = new Size(1920, 1080);

        var tlMain = new TableLayout();


        var trRow = new TableRow();
        trRow.ScaleHeight = true;
        tlMain.Rows.Add(trRow);

        var tcCell = new TableCell();
        tcCell.ScaleWidth = true;
        trRow.Cells.Add(tcCell);

        var btnMain = new Button();
        btnMain.Text = "This button fills the entire form";
        trCell.Control = btnMain;

        Content = tlMain;
    }
}

HalfFilledForm.cs

partial class HalfFilledForm : Form
{
    void InitializeComponent()
    {
        Title = "Test Form";
        ClientSize = new Size(1920, 1080);

        var tlMain = new TableLayout();


        var trRow = new TableRow();
        trRow.ScaleHeight = true;
        tlMain.Rows.Add(trRow);

        var tcCell = new TableCell();
        tcCell.ScaleWidth = true;
        trRow.Cells.Add(tcCell);

        var btnMain = new Button();
        btnMain.Text = "This button occupies the left half of the form";
        tcCell.Control = btnMain;


        // adding another cell to the table row and setting both cells' ScaleWidth property to true evenly divides screen spaces between the two of them
        // you may add more cells in front and behind of your "main" element as spacers.
        var tcSpacer = new TableCell();
        tcSpacer.ScaleWidth = true;
        tcSpacer.Control = null; // <-- set the content of a cell as null and set ScaleWidth as true to create empty spaces with TableLayout
        trRow.Cells.Add(tcSpacer);

        Content = tlMain;
    }
}