The "virtual machine" is not what you would probably consider a "VM" from the usual use of the term, like a "virtual x86/ARM computer" that you can do anything with.
In this case, it's simply a set of variables used to simulate CPU registers, an instruction set that's specific to the underlying database's data format and desired operations, and then some code that "virtually" emulates what a CPU might do if it were designed with this memory architecture (the database file) and instruction set. The VM supports primitive operations via the instruction set; liken this to the machine code for your laptop's CPU, but very specific to the use-case of a SQLite database. The SQL you send into the SQLite db transaction is compiled into this VM's "machine code" and then run on the "virtual CPU" that implements that instruction set, and then it spits out your results. The article was very clear on how it worked.
In this case, it's simply a set of variables used to simulate CPU registers, an instruction set that's specific to the underlying database's data format and desired operations, and then some code that "virtually" emulates what a CPU might do if it were designed with this memory architecture (the database file) and instruction set. The VM supports primitive operations via the instruction set; liken this to the machine code for your laptop's CPU, but very specific to the use-case of a SQLite database. The SQL you send into the SQLite db transaction is compiled into this VM's "machine code" and then run on the "virtual CPU" that implements that instruction set, and then it spits out your results. The article was very clear on how it worked.