show cutomtext (custom field) in front-end product page

You could create a custom fields of the table "item" and of the type "custom text" for that.
I don't understand why you want to use coding instead ?
If you want to add that with code, you would have to edit the file "show_block_custom_item.php" via the menu Display>views.

Hi,

I saw your screenshot already.
Unfortunately, your new message doesn't give any new information so I can't see what I can say besides what I already said.
If it's not your request, please reexplain with more information what you want to do. And maybe why a custom text custom field wouldn't do it ? And why a customization of the view wouldn't do it ?

Hi,

You need to create a custom field of the type "custom text" for each group header and enter the name of the group in the "custom text" field of the custom field.
Then, make sure that you order it just before the fields of that group.
Then it will display similarily to the screenshot in your first message.

Hi,

How are your other custom fields configured ?
If they are custom item fields, then you want to use a custom field of the table "item" and it will work fine on the product page on the frontend.
If they are custom product fields, then you can't use a custom text custom field because they only display on the form (where you can fill in the fields), and thus only show on the backend product edition page. So in that case, the only solution would be to edit the file "show_block_custom_main.php" via the menu Display>views to add the display of the custom text custom fields.

Hi,

Well, ti's quite easy. You just add such code after the foreach line:
if($fieldName=='XXX') echo '<tr><td>YYY</td></tr>';
where XXX is the column name of the first field of the group and YYY the text you want to display before the group.