Electronic – Using attenuators to damp reflections on transmission line

An interconnection should be considered a transmission line if reflections are detrimental to your application. As Leon Heller says, 1/10th wavelength is a good rule-of-thumb.

However, all this depends on the nature of the signal (1/10th of what wavelength?). With sine waves, the wavelength in the cable will be about $\frac{2 X 10^8}{f}$ where f is the frequency, but will vary according to the cable type. The effect of reflections on sine waves is to cause standing waves, so the amplitude will vary along the length of the cable but will always be sinusoidal. With square waves the situation is complicated by harmonics. Although the square-wave may have a low fundamental frequency, the harmonics introduced by the edges will be significant. The effect of reflections in this case is undershoot, overshoot or ringing depending on where the mismatches are (if both $R_S$ & $R_L$ are lower than $Z_0$ for example, you will get ringing).

So do you need to match the source? No. As long as the load is perfectly matched, the wavefront will travel down the transmission line and be fully absorbed by the load. Consider a digital system with 3.3V logic for example. If you terminated both source and load you would only get 1.65V swing at the load. Terminating only at the load is termed parallel termination.

There is also a technique known as series termination in which the source is terminated but the load is left open circuit. In this case, a half-amplitude edge propagates down the line and is reflected back to the source, lifting the voltage to full amplitude as it returns until it is absorbed by the source termination. Along the line, you would see a 'stepped' signal except right at the load where you would see a clean waveform.

Resistors are about the least efficient way to build a splitter. A resistive splitter will split the current and waste the excess voltage. So for a 2-port resistive splitter you lose 6dB. They do have some advantages in specialist situations but practically they aren't used much (I found when I needed one I had to build it myself).

Afaict low loss TV splitters are based around transformers. The transformers can match the impedances without too much loss and can maintain reasonably consistent behaviour over a very wide frequency band. A transformer based splitter built out of ideal components would have a 3dB loss but real transformers are not ideal, so 3.5dB is about as good as you can get.

Mini circuits (a maker of high quality RF splitters) have an appnote describing how transformer based splittlers work. https://www.minicircuits.com/app/AN10-006.pdf

Transmission line or inductor/capacitor based splitters (like the Wilkinson one mentioned in another answer) can be even more efficient (real transmission lines and simple components have less loss than real transformers) but they only work over a narrow frequency band so are not suitable for TV (there is approximately a factor of 2 frequency difference between the top and bottom of the UHF TV band, far more if you include VHF).