The transformer doesn't have an impedance of its own; it only reflects a voltage (or impedance - just the square the turns ratios) from the primary winding to the secondary windings through a ratio of the number of turns of the secondary compared to the primary. Magnetic flux linkage in a ferromagnetic material due to Hopkinson's Law is why transformers transform voltage across a primary impedance, into AC voltage across a secondary impedance and subsequently into a (well matched) inductive load such as a speaker.
The amount of magnetic flux in the ferromagnetic core is dependent upon current flow within the primary coil of wire, the number of turns on the primary, and the reluctance (magnetic resistance) of the ferrite core of the transformer. That’s why the material matters for the plates that make up a EI or C core transformer. The current flow generated within the coil generates a magnetic field, which causes magnetic flux to follow the path of least reluctance. The magnetic flux then traverses through the ferrite material to the secondary windings; generating a current within the coils of the secondary through flux linkage. Voltage is then produced across the secondary terminals due to Faraday’s law of Induction.
In summary, transformers are passive lossy devices. Power in is power out minus losses. The impedance you’re changing on the secondary are strictly for maximum power transfer, stepping the primary tube impedances down to speaker load impedances. In essence, the transformer takes high voltage swings of transient plate currents from the tubes and transforms (hint, transformer) them into low voltage high current sources for the speakers.
I also want to note that there is a lot of magnetic flux stored in a transformer core - which can cause havoc to circuits when impedances are mismatched (primary OR secondary). This means mismatching secondary speakers to secondary windings reflecting an intended impedance from the plates, which may or may not be correctly matched with tube swaps to a particular transformer’s intended primary impedance, can cause transient responses which over frequency can be damaging at high volumes. Just keep that in mind.
