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@allObjects
Amount of 'Loss/Inefficiency' - measured in 'Efficiency' - depends on how many LiPo cells such power bank includes, and also how much current is drawn.
If it is a two cell, then a buck / step-down-converter, such as ti's TPS562210 is an example...
If it is a single cell, and you need only 3.3 volts, you may get away with a low-drop out voltage (LDO) voltage regulator that you connect AFTER the cell protection circuitry. Do NOT remove the cell protection circuitry, because it prevents silent dead by over-discharge (towards and below 3V), and violent dead with possible collateral damage by over-load and over-charge: fire and possible explosion. Your 'heavy' load you connect directly to the protection circuitry output.
If it has one cell and you need 5Volts, ti's TPS61256 is an example for a boost converter. And same story here: efficiency depends on on load as well.
Also with a multi-cell you can connect right after the protection circuitry and before the converter... Most protection circuitries include protection from both and facilitate in-circuit charging for easy use.
Efficiency is comparable, and quite current consumption is quite low... for buck step-down and boost step-up I attached (in same sequence) example efficiency graphs... For best efficiency, the circuitry is tailored to max and min/nominal input voltage, output voltage and output current drawn.
You can even buy just the power bank case with built-in circuitry and supply yourself the readily available and price worthy standard 18650 cells.
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