The core in the injection transformer is probably some nanocrystalline material as T3sl4co1l pointed out. It's got to be to get the high permiability required to get high magnetising inductance at low frequency. As it's a wideband transformer the low frequency limit is defined by the allowable peak flux density. From the data sheet fig. 5-2 they recommend a maximum drive level of 10dBm at 10Hz assuming a 10ohm injection resistor.

So, 10dBm is 0.707V RMS into 50 ohms or 1.414V RMS open circuit voltage at the generator output. Assume load resistance is 10 Ohms so that gives you 1.414*10/(50+10) or 235mV RMS across the primary. Peak flux density from the transformer equation is Bpeak(T) = Erms/(4.44 * f * N * Ae). N = 40, f = 10Hz, lets be generous and say Ae is 3cm

^{2} = 3x10

^{-4} m

^{2} putting the numbers in gives about 440mT peak at 10Hz. Also in the data sheet they give peak flux density as a volt time product, 3.5E-3 Vs, assuming 3cm

^{2} that gives a peak flux density of around 300mT. In calculating the RMS primary voltage I left out the primary magnetizing inductance and that would reduce the primary RMS voltage a little bit. It seems like quite a high working flux density, compared to ferrite anyway. It's only just about useable at 10Hz and I would say "usable at 1Hz" is pushing it a bit

Bifilar literally means "two filiaments" and it's two parallel wires bonded together, I've got a reel of it somewhere for winding coupling transformers on tiny binocular cores. Scientific wire company sell it

https://www.scientificwire.com/acatalog/bifilar.html The winding on the toroidal core is twisted to probably reduce the electric coupling between adjacent turns, MiniCircuits for example wind coupling and matching transformers using twisted wire.

Where are the mixers ? They're probably using analog switches, cheap, wide dynamic range and poor noise figure.