You need to use separate elements Runfor text written in different ways. You can add bold by creating an element RunPropertiesand adding an element to it Bold.
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using (SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.Open(filename, true))
{
WorkbookPart workBookPart = spreadsheetDocument.WorkbookPart;
WorkbookPart workbookPart = spreadsheetDocument.WorkbookPart;
WorksheetPart worksheetPart = workbookPart.WorksheetParts.First();
SheetData sheetData = worksheetPart.Worksheet.Elements<SheetData>().First();
Row row1 = new Row() { RowIndex = 1U };
Cell cell = new Cell() { CellReference = "A1" };
cell.DataType = CellValues.InlineString;
Run run1 = new Run();
run1.Append(new Text("ABC"));
RunProperties run1Properties = new RunProperties();
run1Properties.Append(new Bold());
run1.RunProperties = run1Properties;
Run run2 = new Run();
run2.Append(new Text(Environment.NewLine + "XYZ") { Space = SpaceProcessingModeValues.Preserve });
InlineString inlineString = new InlineString();
inlineString.Append(run1);
inlineString.Append(run2);
cell.Append(inlineString);
row1.Append(cell);
sheetData.Append(row1);
}