All formatting properties are properties of the cells. So everything
you mentioned can be done through the cell properties:

change font:
Range("A1").Font.Name = "Times New Roman"

bold with regular text:
ActiveCell.Characters(Start:=1, Length:=3).Font.FontStyle = "Regular"
ActiveCell.Characters(Start:=4, Length:=3).Font.FontStyle = "Bold"

Any property can be done in the same way, font, size, bold, color, etc.

So all you have to do is concatenate the string first and then apply
the formatting through the cell.