I would suggest this document:
vatican.va/roman_curia/congregations/ccdds/documents/rc_con_ccdds_doc_20020513_vers-direttorio_en.html
Liturgical prayers are those for which the Church has an official, established, approved form. Liturgy is also universal—it belongs to the whole Church, while other forms (such as popular piety) might apply only to certain cultures, or certain groups.
While that document (the Directory) is mostly concerned with defining “Popular Piety” it can be helpful because by knowing “what is popular piety” we know that it “is not Liturgy” as such.
For the most part, whatever involves the administration of a sacrament is part of the Church’s liturgy.
Also, whatever is directly necessary for the sacraments is Liturgy. Some examples would be the consecration of a new church building, the establishment of a diocese, blessing of a baptismal font, etc. Liturgy concerns what is
necessary for the Church’s spiritual life.
One criteria is that if it applies to the whole Latin Rite Church, and the form is obligatory (ie the rites and ceremonies are clearly defined by Rome), then it’s Liturgy. The reason is that Liturgy belongs to the whole Church (at least the “whole Church”
sui iuris).