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Diffusion to depths happens in a cone-shaped manner, to achieve higher subcutaneous absorption is necessary to apply the drug on a large surface. Cutaneous absorption varies on the body zone:

- higher in: scalp, forehead, mandible, armpit and scrotum (especially this one). Generally it's higher in places highly vascularized and with thin skin

-lower in: palm, plantar arch and ankles. Generally it's lower in poorly vascularized areas and thick stratum corneum

The famous study on hydrocortisone absorption reported skin permeability was: 0.1 for sole and 42 for scrotum (lowest and highest values).

For estradiol:

Transdermal and intramuscular have the highest half-lives (time it takes to reach half of the drug concentration). Transdermal has low bioavailability (for example if you use 100g of product the bioavailability is the fraction of product that reach systemic circulation out of 100g) but its very potent. Also it bypasses the first pass effect (hepatic metabolism). Intramuscular has almost 100% bioavailability, acts fast and has fast duration.

For testosterone:

Transdermal gel has low bioavailability, most permeable areas are thighs/abdomen (hair WILL grow more in the application area). IM injections have a LONG duration and bioavailability is almost 100%.

Oral route is always the worse way to take hrt because of high first pass effect :')

I still have to take my pharmacology exam but I've been studying for it for MONTHS. Anyways, I'm not an expert and these are general information, for more details read about your specific drug :)