How the cells in epithelial tissue maintain difference between environments on either side of the epithelium?

HAI,Samarth mishra
The word epi- represents exterior.
Most of the type of epithelial tissue exists as protective layers by tranforming them selves from one form to the other form.
such as changing themselves from
Stratified epithelium to⇔ Keratinised epithelium to ⇔Complex epithelium
example for better understanding is
1.Secretion of sweat sebum from skin layers
2.Formation of hair on skin (to exchange heat from body)
3.Chage in colour due to melanin secretion in skin cells(Melanocytes)
The above examples are modifications of epithelial cells towards different external environnment.