My experience is that there is a hierarchy in the way fields are filled:
Alt is displayed when the user hovers over a thumbnail.
Alt is populated during image upload from the field identified variously as:
ExifTool fieldname: IPTC > ObjectName
IrfanView fieldname: IPTC > Document Title
Photoshop Elements File Info fieldname: IPTC > Status > Title.
If this first choice field is null, the second choice is to populate Alt from the field used for Title (see next). In this case, Alt and Title will acquire the same data. If the second choice is also null, then the final choice is to populate Alt using the file name with any blanks filled with ‘-‘ or ‘_’ (option)
Title is displayed when the user clicks on a thumbnail and the image is displayed in a pop-up.
Title is populated during image upload from the field identified variously as:
ExifTool fieldname: IPTC > Caption-Abstract
IrfanView fieldname: IPTC > Caption (Description)
Photoshop Elements File Info fieldname: IPTC > Content > Description
If this field is null, the default Title is null.
Tags is populated during image upload from the field identified variously as:
ExifTool fieldname: IPTC > Keywords
IrfanView fieldname: IPTC > Keywords
Photoshop Elements File Info fieldname: IPTC > Content > Keywords.
If this field is null, the default Tags is null.
As noted, you can prefill the EXIF fields as you wish eg, using an EXIF editor tool.