Does hair essence only have a temporary effect?

Lately, my hair feels drier and brittle than before, so I'm worried.
So I was thinking of trying a hair essence, but there were so many types.
There are oil types and cream types, and I don't really know what the difference is.
I'm also curious if applying the essence definitely improves the condition or if it's just temporary.
I'm also confused about whether it's better to apply it before drying or after.
I'm also worried that if I use it wrong, my hair might look even greasier.
It is unclear whether hair products require steps like skincare.
So I'm just debating whether I really need to use it in my current situation.