Different mechanics of hyperlink vs. hyperref

In your question you say

...hyperlink{target}{text} and hyperref[target]{text}...

but it should probably be

...hyperlink{⟨named destination⟩}{⟨text⟩} and
hyperref[⟨cross-referencing-label⟩]{⟨text⟩}...

Don't confuse names of LaTeX's cross-referencing-labels with the names of so called "named destinations"="targets"="anchors" which get placed into the .pdf-file for hyperlinking:

Basically a cross-referencing-label is a record of data maintained via the .aux-files during the LaTeX-run and providing pieces of data holding information that is needed for cross-referencing. A cross-referencing-label exists during a LaTeX-run/exists while the LaTeX-compiler is running and creating the .pdf-file/output-file.

It does not exist when the .pdf-file/output-file that was produced during the LaTeX-run is displayed by a pdf-viewing-application.

The name of the cross-referencing-label is the name of the record. Usually such a record consists of several elements like a page-number and the printed value of some LaTeX-sectioning-counter and - in case hyperref is loaded - the heading of the corresponding section and the name of a "named destination" placed into the .pdf-file for hyperlinking that section/for "navigating" to that section while viewing the .pdf-file. Different referencing-commands can retrieve different elements of such a record.

E.g., with ref the printed value of the LaTeX-counter is retrieved. In case the hyperref-package is loaded, the name of the "named destination" which due to some sectioning-command (section, subsection, ... caption etc) automatically got placed at the beginning of the corresponding item of sectioning is retrieved also for turning the printed value into a hyperlink leading to the corresponding item of sectioning.

I tried to explain the concepts related to LaTeX 2ε's cross-referencing-mechanism in my answer to the question "How to prevent reference to enumeration inside new environment?".

A named destination=a target=an anchor -- as already explained by Ulrike Fischer -- is an object which gets placed into the .pdf-file itself and which is used by the .pdf-viewing-application for "navigating" to a specific "place" of the document when viewing the .pdf-file.

A named destination/a target/an anchor does exist when the .pdf-file/output-file is displayed by a pdf-viewing-application.

When loading the hypperref-package, then LaTeX does automatically place such objects/named destinations into the .pdf-file when processing sectioning-commands like section, subsection, ..., caption. Due to the label-command (which triggers writing to the .aux-file the record of data which you wish to use for cross-referencing) LaTeX beneath other components also stores as a component of the cross-referencing-label the name of the named destination which was placed automatically by LaTeX as the last one before encountering the label-command in question. Referencing-commands like ref or pageref can extract this piece of data for turning things into hyperlinks.

Besides hyperref's automatic placing of named destinations you can use hypertarget for placing a named destination into a .pdf-file "by hand".

The difference between hyperref[sectionheading]{some text} and hyperlink{section.1}{some text} is:

The optional argument of hyperref[sectionheading]{some text} denotes a cross-referencing-label and the hyperref-command will wrap the phrase "some text" into a hyperlink by obtaining the name of the corresponding named destination from that cross-referencing-label which basically is a record of data whereof one component denotes the name of a named destination that got placed into the .pdf-file.

The first non-optional argument of hyperlink{section.1}{some text} directly denotes the name of a named destination that got placed into the .pdf-file. The name of the named destination is given directly and is not to be obtained as a component from a record of data handled as cross-referencing-label.

If you like it cumbersome you can use the refcount-package for obtaining single components of these records of data that are called cross-referencing-labels and do something like this:

documentclass{article}

usepackage{pdfpages}
usepackage{hyperref}
usepackage{refcount}
...
section{section}label{sectionheading}
... 
IfRefUndefinedBabel{sectionheading}{%
  refused{sectionheading}%
}{%
  hyperlink{getrefbykeydefault{sectionheading}{anchor}{UndefinedDestination}}%
            {some text}%
}
...

With this construct during the first LaTeX-run, i.e., while cross-referencing-labels aren't recorded to the .aux-files yet and thus still are undefined, refused{...} will be carried out and you get questionmarks into the .pdf-file and warnings into the .log-file and on the console.
In consecutive LaTeX-runs the name of the named destination belonging to the cross-referencing-label sectionheading will be extracted from the data-record formed by the cross-referencing-label sectionheading and delivered to the hyperlink-command by the getrefbykeydefault-command.

But you don't really need this. hyperref[sectionheading]{some text} is shorter and does the same. ;-)

Links go to destinations. Destinations are objects in the pdf created (in pdflatex, for the other engines they are similar commands) with the primitive pdfdest command. The destination object contains a coordinate and a page object reference number. In a different part of the pdf there is a reference from the name of the destination to this object.

<<
/D [6 0 R /XYZ 133.768 667.198 null]
>>

hyperref creates a number of such destinations automatically.

For example a section command creates a destination called section.1 , section.2 etc.

You can also add more destinations with the hypertarget command.

To create a link to such a destination you can either use the name of the destination directly (if you know it) with hyperlink and a brace argument:

hyperlink{section.1}{some text}

Or you can add a label to the structure where hyperref adds internally destinations and then use hyperref and the bracket notation, then hyperref will find the destination name by going through the label information:

section{section}label{sectionheading}
hyperref[sectionheading]{some text}

Your third variant does not work:

hyperlink{sectionheading}{some text}

It gives a warning in the log:

pdfTeX warning (dest): name{sectionheading} has been referenced but does not 
exist, replaced by a fixed one

That means that you get a link, but not to the right place as the destination sectionheading doesn't exist (the "fixed one" used as replacement is normally the first page.)

includepdf creates a number of destinations for the pages called (in your example) pdfpage.1, pdfpage.2 etc. So to link to this destinations you have to use the brace notation:

 hyperlink{pdfpage.1}{hyperlink}

(To make life difficult, hyperref accepts also four braced arguments to link to an url, and I at least gets constantly confused by this).