Many of the receptors for exogenous substances have other functions.
The KEGG site is one of many open sourced sites for scientists, and is frequently referenced in the scientific literature. There are many others. (I came across a reference to it in a paper I was reading this evening, which caused me to throw this post out.)
I often use, professionally, the Uniprot database, which is linked to some of the software utilized in our lab's mass specs.
Here is a list of structures of cannabinoid receptor proteins found on that website: UniProtKB 50,481 results or search "cannabinoid" as a Protein Name, Gene Ontology, or Disease
Note that these receptors do not exist because of cannabis. They have other functions, but cannabis acts on them.
The metabolism of exogenous substances, like, say, THC, is controlled by a particular class of enzymes known as "CYPs"
Ulrich M. Zanger, Matthias Schwab, Cytochrome P450 enzymes in drug metabolism: Regulation of gene expression, enzyme activities, and impact of genetic variation, Pharmacology & Therapeutics, Volume 138, Issue 1, 2013, Pages 103-141. These enzymes are very flexible and can catalyze metabolism of a wide range of exogenous substances. They are very important in pharmacology and are much studied.
One of the positive things about the development of the internet is the availability of a wide array of databases for use in a wide array of sciences. These are the best of times and the worst of times.
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