As the question implies, I'm interested in only the semantical aspects of the two words I've listed. I've looked up these two words on some online dictionaries. After some searching, I've noticed that the third definition of nexus and the first/second definition of locus are nearly identical in meaning.
Cambridge Dictionary lists the definition of nexus as "an important connection between the parts of a system or a group of things"
Whereas locus is defined by the same online dictionary as "the place where something happens or the central area of interest in something being discussed:"
Here are some examples to expound my point:
Example 1: For us, the island had become the locus of the struggle.
Times Square is the nexus of the New York subway.
Example 2: The real nexus of the money culture was Wall Street.
The community is a locus of healing, not the hospital or the clinic.
In the two previous examples, my brain's understanding of these two words has led me to perceive them as sharing the definition of "a place or locality as the cause or source of great activity".
Best Answer
Thank you for researching your question so carefully. To the extent that there is a difference between these two words,you can most clearly see the difference by looking to their derivations.
Both are derived from (or simply are) Latin words. The Latin locus means a place. You might think of it as a dot on a map. In Latin nexus is also a noun, meaning a 'bond'. It, in turn, is derived from the verb nectere (past participle nexus) meaning to bind or weave. The point about nectere is the it involves joining things together (threads, pathways, journeys... networks). And yes, it seems that even the word net has the same derivation.
So Time Square in New York, Picadilly Circus in London, Charles de Gaulle Airport in Paris are both loci (the plural of second declension locus in Latin) and nexūs (the plural of fourth declension nexus pronounced nexoose, I'm sorry to say, because necto is a third conjugation verb!). That is they are places and they are places where several journeys by car, foot or aeroplane meet. And you can describe the networks themselves as nexūs (but don't worry, no one will stop you from calling them nexuses, just avoid nexi in case some pedant like me is standing nearby ready to judge).
So which of the two words you use is really a matter of which aspect you wish to emphasise, the point location or the intersection of different threads.