There are 2 types of tags when it comes to EMV. EMV standard tags and ID TECH proprietary tags.
For EMV standard tags... They are defined and documented by EMVCo. The link below is a convenient way to search for the tags you need either by tag number or by keyword. For ID Tech proprietary tags, I will include a link to the latest Tag Reference Guide. As for how to retrieve these tags. This is best demonstrated in the EMV Transaction in Universal SDK document linked below. Pay particular attention to the section: 2.5.3 Obtaining Extra Tags