Jets powered by active galactic nuclei (AGN) are some of most powerful phenomena of the cosmos. Understanding the underlying physical mechanisms is necessary to enhance our knowledge of the universe. The focus of this thesis lies on the innermost region of the radio source 3C 84, harboured in the radio galaxy NGC 1275, which exhibits such energetic jets. Perhaps connected to this jet activity is a perpendicularly to the bulk jet flow oriented structure, which was recently revealed in a RadioAstron space very-long-baseline interferometry (VLBI) image. Interpreting this structure and its implications to jet physics is one of the main motivations of this thesis. It is accomplished by utilising millimetre-VLBI observations of 3C 84 at the highest resolution, with the available data covering a period of more than twenty years and at different frequencies. The thesis is organised as follows: in Sect. 1 an overview of the astrophysical background required to interpret the data is introduced. This includes brief descriptions of the radiation received by the telescopes, of black holes and AGN, as well as of astrophysical jets. AGN classification schemes and the relevant details of jet physics are also discussed. Section 2 offers a review of the technical background, including the basics of the technique of interferometry and VLBI arrays, calibrating a VLBI data set and imaging it. In Sect. 3 we utilise quasi-simultaneous observations at 15, 43, and 86 GHz and create the highest resolution spectral index images of 3C 84 to date. Our analysis reveals the existence of a spectral index gradient in the north-south direction, with values between $\alpha_{43−86} \sim 2$ upstream of the 86 GHz VLBI core and $\alpha_{43−86} \sim -2$ downstream. In this context, we discuss the spectral index distribution. We determine the location of the jet apex to be 400 − 1500 Rs (Schwarzschild radii) upstream of the 86 GHz VLBI core, by means of two-dimensional cross-correlation analysis. In that region, the magnetic field appears to be a mix between poloidal and toroidal, with a strength of 2 − 4 G. Section 4 presents an alternative approach for pinpointing the jet apex of 3C 84, by directly imaging the core region. The temporal stacking of a number of 86 GHz data sets at different epochs confirms the existence of a double component structure present in the core region, concurring the RadioAstron result. Both a conical and a parabolic jet expansion profile are then fit to the data to determine the shape of the expansion. This constrains the position of the jet apex to 200 − 3000 Rs upstream of the 86 GHz VLBI core. Our analysis also reveals a possible change of viewing angle along the jet flow (perhaps indicative of jet bending) and sets an upper limit for the viewing angle of 35 degrees for the inner jet. Section 5 showcases a comprehensive study of the evolution and jet kinematics of 3C 84 over more than twenty years. Our analysis reveals the ejection of numerous components from the core region, which seem to move at subluminal speeds, with newer components being faster. We also checked for possible differences between the velocities of the 43 and 86 GHz components individually but only found marginal evidence of faster motion at 86 GHz. The jet width appears frequency dependent, with the jet width decreasing with in- creasing frequency, which might be explained by stratification in the context of the spine-sheath jet stratification scenario. We also produced spectral in- dex maps at 43 − 86 GHz, which show that the orientation of the spectral index gradient position angle is time variable. This further indicates that the black hole is positioned off-centred from the total intensity maximum and that the jet axis is changing with time. Finally, in Sect. 6 our analysis and results are summarised and in Sect. 7 an outlook for the future is provided.