OVERALL: The first page of this topic has a LOT of good information about background screening, including the formal criminal and security checks conducted by the RCMP and CSIS respectively.
The link recently posted by
@caipsnotes is also a good resource even though it is mostly about CSIS screening.
REMINDER: "Background check" is another one of those terms sometimes used in a specific sense, typically in reference to the the RCMP criminal history check and the CSIS security check, BUT is also used in a more generic sense in respect to the verification and assessment process which is done by IRCC processing agents. The former is often used in the context of WAITING for background check (or background clearances), which is a specific reference to waiting for the RCMP or CSIS clearance or updated clearance. But, for example, when an applicant is informed by a telephone agent in the Help Call Centre that their application is waiting for "background" clearances or checks, that is not informative, since IRCC must do another GCMS background screening before taking any action on a citizenship application, so the application is ALWAYS waiting for at least one more background check right to when there is a final Decision Made. All the Call Centre is actually saying is that the application is in queue waiting for the next task to be undertaken.
Again, there is a good deal of information about the nature and scope and quite a few specifics about "background" checks or screening on the first page of this topic. While those posts are more than two years old, most of that information is still valid.
You are correct that the RCMP check is "more thorough" in regards to the FOUR years (not five) prior to the date the citizenship application is made. This is specifically about screening for criminality related prohibitions, which is about criminal charges or convictions within the FOUR years prior to the date of the application.
We know less about the scope of the CSIS screening (regarding this, the link provided above by
@caipsnotes offers some good detail which on its face appears credible), but it very likely is "more thorough" in regards to updating screening since the last security screening, which for most PRs applying for citizenship this means since becoming a PR. For the vast majority of citizenship applicants the CSIS screening is minimal, resulting in a "No Reportable Trace" (NRT) report. Indeed, I'd venture this is the outcome of the CSIS screening during the PR process for the vast majority who become a PR, and updating this for citizenship has NO impact on the processing of the citizenship application at all . . . these checks are done collaterally while the citizenship application proceeds through its processing by IRCC.
However, citizenship applicants who have lived or worked abroad since becoming a PR may be screened more thoroughly than those who came to settle in Canada and have stayed since landing (allowing for typical travel abroad of course). It is possible that CSIS (again, there is a lot about what CSIS does that is not public information, a lot we do not know) is directly engaged in screening citizenship applicants for criminal charges or convictions abroad, since the citizenship prohibitions were revised to include criminality prohibitions for foreign crimes; however, it is more likely that IRCC conducts such screening through its visa offices abroad. Again, this would be focused on the FOUR years prior to the citizenship application, the relevant time period for criminality related prohibitions.
More Regarding "Background" Screening in General:
What many discussions about background screening underestimate is the nature and scope of IRCC's own screening processes. Here too, much of this is not public information, categorized as "confidential," which in effect means secret (albeit with wider access to it inside the government than formal "secrets"). But over the years many of the citizenship cases that have gone to the Federal Court indirectly reveal some of the nature and scope of what IRCC does. For example, we know that IRCC sometimes accesses "open sources" and thus will, in SOME instances, research social media regarding an applicant, including in particular sites like LinkedIn. We know as well that sometimes citizenship applicants are telephoned (I was) and it readily appears that IRCC can telephone various sources of information, including for example current or former employers. It appears the nature and scope of these inquiries can vary considerably from one applicant to another, perhaps minimal scrutiny for many if not most, but rather extensive and even intensive scrutiny for some . . . which makes sense, since in almost all immigration related screening, what processing officials typically do is follow a triage protocol pursuant to which there is minimal follow-up for most but for more than a few this or that factor triggers more inquiries, and what is learned from the results of those inquiries will either satisfy the official or lead the official to pursue additional inquiries.
The vast, vast majority of QUALIFIED citizenship applicants who properly followed instructions and provided complete and truthful information, and who have no criminality or security issues, have NOTHING to worry about in regards to background screening in any of its respects. Contrary to some assertions otherwise, background screening rarely affects the processing timeline. It is not, not for the vast majority of applicants, a cause for processing delays.