Over 80,000 chemicals comprise the so-called “terpenome” class of the natural product family.  Considering such a massive group of natural chemicals that demonstrate considerable chemical, structural, and functional diversity, it’s easy to see why this terpene and terpenoid class is of particular interest to researchers in the pharmaceutical, food, cosmetic, and agricultural industries. 
All plants produce terpenes, but the kinds of terpenes will vary based on the taxa as well as the ecological conditions that may cause the terpenes to adapt and evolve. The basic pathway through which the terpenes are synthesized is the similar across terpene groups, however, recent research has demonstrated variations on the types of substrates and enzymatic reactions that can be involved in terpene synthesis. 
The basic building blocks of terpenes are two C5 precursors, aka isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). The terpene synthase (TPS) family of enzymes catalyzes the biosynthesis of terpenes. Specifically, isopentenyl phosphate kinase (IPK), which is found in all sequenced plant genomes, is responsible for the reaction that transforms isopentenyl monophosphate (IP) and dimethylallyl monophosphate (DMAP) into the aforementioned C5 precursors IPP and DMAPP, respectively.  Researchers have been interested to understand the origins of IP and DMAP.
Traditionally, IPP and DMAPP act as substrates to TPS enzymes that remove the diphosphate group through head-to-tail reactions to form the “backbone” of the major terpene groups: monoterpenes, sesquiterpenes, and diterpenes.  Research has now revealed that terpenes can also be synthesized through non-TPS pathways that involve unusual condensation (irregular non-head-to-tail) reactions.
For example, the synthesis of the monoterpene alcohol lavandulol in lavender involves a head-to-middle irregular condensation of two DMAPP molecules to create the precursor lavandulyl diphosphate. This reaction is catalyzed by lavandulyl diphosphate synthase. 
Terpenes may also be synthesized through catalytic reactions that involve microbial TPS. The TPS in bacteria and fungi have shorter proteins comprising mostly of an α domain (plants typically have two or three domains, α, β, γ). 
In summary, terpene diversity exists in the substrates from which they are derived and the enzymatic reactions that take place during their biosynthesis. Being such a diverse class, the complexities involved in their creation have limitless possibilities which researchers are yet to uncover.
References Christianson DW. Structural and Chemical Biology of Terpenoid Cyclases [published correction appears in Chem Rev. 2018 Dec 26;118(24):11795]. Chem Rev. 2017;117(17):11570-11648. doi:10.1021/acs.chemrev.7b00287 [journal impact factor = 60.62; times cited = 349]  Pichersky E, Raguso RA. Why do plants produce so many terpenoid compounds?. New Phytol. 2018;220(3):692-702. [journal impact factor = 10.151; times cited = 180]  Zhou F, Pichersky E. More is better: the diversity of terpene metabolism in plants. Curr Opin Plant Biol. 2020;55:1-10. [journal impact factor = 7.834; times cited = 47]  Henry LK, Gutensohn M, Thomas ST, Noel JP, Dudareva N. Orthologs of the archaeal isopentenyl phosphate kinase regulate terpenoid production in plants. Proc Natl Acad Sci USA. 2015;112(32):10050-10055. [journal impact factor = 11.2; times cited = 74]  Chen F, Tholl D, Bohlmann J, Pichersky E. The family of terpene synthases in plants: a mid-size family of genes for specialized metabolism that is highly diversified throughout the kingdom. Plant J. 2011;66(1):212-229. [journal impact factor = 6.417; times cited = 753]  Jia Q, Köllner TG, Gershenzon J, Chen F. MTPSLs: New terpene synthases in nonseed plants. Trends Plant Sci. 2018;23(2):121-128. [journal impact factor = 11.39; times cited = 31]