An algorithm which is only known by A has been checked in. Can B ask clarifications in the code review?
The external behavior of the algorithm should be documented with comments it it is not obvious from just reading the signature. If the internal implementation is complex it should likely also be documented, possibly by referencing wikipedia, some research paper, or other source. So B should not just ask for clarifications, but ask for these clarifications to be added to the code.
It is possible, and even likely, that developer C will be asked to do something with the code many years after it was written, long after both A and B has left the company. So ensuring readability is very important.
A third party library has been used for a specific function f and some code using it has been checked in by A. Can B ask why is f useful? how about more details about f such as "how does it internally work?".
You should not expect developers to know about the internal behavior of all third party libraries they use. So if asking about this you might want to make clear that an answer is not expected. The external behavior of the library should be documented, so hopefully B should be able to check why it is useful himself. But if there is anything unclear, ask for clarifications.
Keep in mind that it is really difficult to determine if an unfamiliar algorithm is implemented correctly just by reading code. This should instead be proven by unit tests whenever possible. So the code review might instead focus on checking if the code is readable and understandable, code standard is followed, if there are typos or other simple mistakes etc.
Also keep in mind what knowledge is expected from your developers. If you are writing a graphics engine it is reasonable to expect knowledge about matrices, vectors etc. But if the domain is narrow you should likely require a higher standard of documentation, since you might not be able to find a replacement if your domain expert leaves. This is especially important when using code from researchers, since there might only be a handful of experts in their particular field in the world.