I posted this article reference as a good example of a serious paper describing an interesting result from a huge experimental effort.
I appreciate that IOP has put the Astrophysical Journal into the open (no paywall), so these papers are readily accessible.
This is an astrophysical paper: it contains physics (specifically, results of General Relativity) and astronomy (specific description of an astronomical object).
Unfortunately for me, the results are quoted in terms of astronomical measurement units (not SI), so I need to look into them before I can better understand the results.
I remember a lecture (ca. 1976) by Edward Purcell, after he started work on interstellar dust: he said that his first task was to calculate the conversion factor between magnitudes/parsec and dB/light-year.
If you look at the full list of acknowledgements, you will see a very large worldwide group of institutions and funding sources, including countries such as China and Taiwan that differ politically.
Section 1 has a good background for the theoretical and experimental results that preceded the group effort (six locations from the South Pole to Spain and Arizona at the north, total of eight machines).
Section 2.1 discusses the properties of "Sgr A*" itself.
Section 3 discusses the observation systems, basically long-baseline interferometry at two bands around 227 and 229 GHz, and the data processing.
Essentially, a metric shitload (modern technical term) of data was loaded onto hard drives, which were then transported to a central location for image reconstruction.
Modern imaging systems, such as CT and MRI, also "reconstruct" the image from multiple measurements (e.g., "projections" for CT), and this is another mature field of mathematical physics or engineering.
In Section 7, the implications of these results for General Relativity and related theories are discussed.
Note that the "Kerr metric" is the GR description for a black hole with angular momentum, which has interesting differences from the "Schwarzschild metric" fir a non-rotating object.
Simply put, a rotating black hole has two event horizons.
An earlier paper, cited in this one, describes the equipment in detail, in conjunction with the earlier measurements on M87*.
https://iopscience.iop.org/article/10.3847/2041-8213/ab0c96/pdf