First of all, one doesn't really "practice a Tantra." One practices a sadhana, based on a tantra, or tantras.
The Tantras are texts, first and foremost. These texts deal with a variety of topics--but not every Tantra addresses every tantric topic. One can find the basic "framework" for a variety of sadhanas in one Tantra for instance, sometimes various forms of one deity, or even different deities entirely, in a single Tantra. Or one can find a detailed explanation of common "stages" of visualization for various sadhanas in a single Tantra, without that Tantra focusing on a single deity. These Tantras are called "general," "shared," or "explanatory" Tantras. Broadly speaking, the stages of Creation and Completion are what is addressed in the Tantras, but those two stages are huge subjects, with many branches.
As an example, the famed Six Yogas of Naropa are, in reality, a collection of completion stage practices derived from a variety of Tantras. "Tummo" is said to derive from Hevajra and Chakrasamvara Tantras, while Illusory Body is derived from Guhyasamaja Tantra, etc. So, if one is practicing in this system, one is working with topics from a variety of Tantras.
Sadhanas themselves, which may have creation and completion stages more or less explicitly written out, or explained in their "Tri" or "explanation," or not, were formulated by various Mahasiddhas in reliance on the Tantras themselves, often in combination with the Mahasiddhas own personal experiences and understanding. A single deity--Vajrakilaya, for instance--can have many different sadhanas, from multiple-headed and multiple-faced (like the Jangter Kilaya) to the more common three-headed/faced, six armed one (Dudjom Phurba, Chogling Phurba, KhonLuk Phurba), to a single-faced two armed one (Khenpo Jigphun's Phurba).....
The practices one undertakes, therefore, relate more to a given lineage, stemming from a Mahasiddha or Terton, but the theory or philosophy underlying the practices can be found in the Tantras themselves.
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