Copyright covers the specific/physical embodiment
Copyright does not cover a concept, nor idea.
Using resources that you downloaded such as images and sprite sheets will be copyright infringement. Unless those items are in the public domain, or are exempted via fair-use doctrine (see below).
Devices, Code, Expression...
Reverse engineering software is not by itself copyright infringement, though there are some cases relating to "look and feel" that might apply to certain features of software or a page but look & feel case law seems to get weaker every year. In general software is a "device/process" which is more likely to be covered by a patent, not copyright.
Certainly the specific code for software as written falls under copyright protection. That is, a specific app as purchased, not your "version" of it if you wrote it from scratch.
For a website, copyright applies to the expressive content, and likely applies to some of the the actual code as written for a page (such as a CSS style sheet, and perhaps some specific HTML, classes, scripts). But there is a lot of "common" code that is the same for many sites, and is not expressive nor unique, and thus is not protected by copyright.
The Facebook logo is covered by trademark, not copyright. Trademark is for items used in commerce, just as a logo or slogan. The rules regarding trademarks are a little different than copyright. For instance you can not copyright a title or a name, but you can use trademark protection if it is used in commerce.
For instance, the title "STAR WARS®" is protected by trademark, not copyright.
Unique or Eunuch
The overall design of a facebook page is not particularly unique — compare the functional elements to the page here at StackExchange. Top sticky bar, left column, right column, center content... That kind of common general layout is not subject to copyright protection.
FB can't say, for instance, that they have the exclusive right to make their sticky bar 43px high and #4267B2. However, the color can be part of their trademark which features other design elements that make it unique.
So, except for the images you pirated, you could build a page that looks the same using all your own code without infringing copyright, but if you copy a "functional element" that is covered by a patent, that might infringe a patent claim.
Also if you copy and paste code, that could be an infringement, just as if you download and use images and sprite sheets will typically be infringement (unless they are PD or under fair use).
Fair Use Sets You Free
Nevertheless, for your intended purpose of instruction/demonstration, you are pretty much falling under the umbrella of fair-use. And provided you don't actually distribute the images you downloaded, fair use should allow them to be used for example/demonstration for educational reasons.
Nevertheless, instead of using the facebook LOGOs, I suggest using parodies of them (you know, like FACEBLOK or FARCEBOOK or FA$ETOOK.)
Here is an analogy using a science fiction story:
A young man is on a desert planet, and finds himself battling a galactic evil empire. He learns that his father was killed by evil factions and vows to avenge his death. He joins rebellious forces, and eventually leads them to a big victory.
I am of course talking about DUNE by Frank Herbert. What story did you think I was talking about?
In expressive writing, there are often many similarities. Plots cannot be copyright protected. Neither can fonts as used in a design or layout (though a font file can be protected). Some other aspects of protectable or not:
- A live performance cannot be protected. But a video OF the live performance can.
- A mere arrangement of "facts", and facts themselves, can NOT be protected.
- Copyright is about a specific embodiment and expressive content in a physical form (digital included).
- A copyright owner has rights & control to their expressive work, including use in derivative works.
- Fair-use allows some limited use for education, news, critique, and parody.