There are many classes of filters. Passive-Active, Lumped-Distributed, Analogue-Digital. Although a filter of any type can be theoretically designed at any frequency, the fact is that in practice the frequency of usage determines the techniques to be used. An audio filter using microstrip would be rather large, likewise a 12GHz bandpass filter would be difficult ( but not impossible) to design using active amplifier design methods.
Microwave filters, due to the distributed nature on the design, require specialised design techniques.
In both RF and microwave engineering it is difficult to go far without needing a combiner to sum various power scource. A 20Kw Band 11 transmitter would typically sum between 20 and 24 amplifiers depending on the market (fiability and robustness decision)
Some aspects of RF engineering need to use microwave techniques. General microwaves are from 1GHz upwards but that depends on what is the customary band of usage. For a habitual of Ku band the L band is "low frequency". Here the concept is the component/circuit is or becoming distributed, ie: the element is larger than 0.1 λ
When a component/circuit becomes distributed, it means the frequency becomes a major factor of the the characteristic. A simple PCB track becomes a transmission line.