Hi you,
#1. Edit booking form: You can copy file themes/triply/template-parts/booking/single/booking_form.php and paste to child-theme, then edit this file.
Editing this booking form is very complicated because it also involves js in the plugin, so be careful when making any customizations in this booking form.
#2. Edit form checkout: You need to copy file themes/triply/template-parts/booking/checkout/information.php and paste to child-theme, then edit this file.
Use “babe_checkout_before_contact_fields” to add filter data to $args field.
The link you sent is a guide to add custom fields for single booking tour