Even though a single chord is often enough to modulate, we frequently find ourselves facing pieces that carry out the modulation process over very extensive periods of the composition; this is because there are needs that go beyond modulation itself, such as "rhetoric," the desire to create a compositional discourse that is rich, extensive, and interesting.
Usually, one refers to the home key, and while it is true that in a classical sonata form there is a strong opposition between the home key and the dominant, the development is the moment when the material traverses (a term not used by chance) harmonic paths that are more unstable and, why not, more distant from the home key. On average, the classics move within the circle of fifths, but as a good pre-romantic, Beethoven begins to move away a little. The techniques to achieve this "deviation" are various, but the problem is doing it "a la maniera de".
Beethoven, from a didactic point of view for learning how to use sonata form, is probably the most difficult because he does not follow a recurring pattern and, in fact, he starts by using the sonata form only to then surpass it; in between lies the world.
It is also true that Op. 18 was composed between 1798 and 1800, a period to which it seems you are interested.
Aside from Op. 18, it wouldn't hurt to take a look at the first two symphonies and about twenty piano sonatas, without overlooking the 3 piano and orchestra concertos, some trios, and the violin and piano sonatas. You will see that the need to expand formal structures and enlarge compositional proportions is evident; he also skillfully develops the composition for violin, to which he entrusts brilliant passages with double strings, triple strings, etc., and assigns equal importance to both the piano and the violin (see the Sonata in F major Op. 24 "Spring" for its lyrical/sentimental vein, and the Sonata in C minor Op. 30 No. 2 for its dark and heroic tone).
Beethoven's compositional development is extraordinary, as demonstrated by the 32 piano sonatas created from 1795 to 1822—pieces that present expressive, compositional, and technical innovations favored also by the perfection of the instrument's mechanics. The early piano sonatas approach the formal models of Haydn, Mozart, and Clementi, although rich, brilliant sonorities and great thematic expansion are already perceptible with the addition of a fourth movement (minuet or scherzo). The first movements in sonata form are large in scale, with expositions rich in contrasting thematic material, dynamic effects, and significant rhythms (see the sonatas: "Pathétique," which begins with a slow movement followed by contrasting themes); in the slow movements, moments of lyricism are present, followed by dark sonorities and particularly expressive atmospheres. The sonorities have different characteristics based on formal construction; for example, Op. 26 begins with an Andante with variations, and Op. 27 presents itself as a quasi-fantasy sonata where freer ideas derived from improvisation are present, such as the Adagio sostenuto of the C# minor sonata, later named "Moonlight" for its dreamy character.
The string quartets and piano Op. 18, published in 1801, are influenced by Haydn, although some formal solutions are already present, such as final movements written in sonata form or sonata-rondo form. The First Symphony in C major Op. 21 was written at age 30 and follows the formal model already adopted by Haydn and Mozart, although the novelty Beethoven introduces is given by the exploitation of technical and expressive resources in all orchestral sections and also by the broken style of the melody that runs between the different instruments of the orchestra.
These are just some elements for reflection to better respond to
http://www.pianoconcerto.it/forum/index.php?/topic/4106-imitazione-stilistica-procedimenti-pratici/
In particular, in the absence of a specific treatise on a certain language, one must approach the issue as if you were writing a stylistic synthesis yourself. Certainly, understanding a composer's style deeply is of great utility, but it is a demanding task; consider that old traditional courses led to writing a fugue (and not always a credible one) after 7 years; after many years of study, some times are shortened, but it remains an arduous task to write well and in a credible way in poetics that are different and distant (even in a temporal sense) from us... since there was mention of "historical forgery."