based on your answer,

1-so by telling me to enter duration as 4 days, you are in a way telling me that, the first time I enter duration for an activity, I should first think of it as if it will be done by just one (1) full time resource? such as i should calculate "how long would it take for just one full time resource to finish this", and then i do the math myself outside, and come up with 4 days and enter that?

2-as soon as I assign 4 days, and then enter just one (1) full time resource, the project automatically multiplies 4 days by 8 hours (because lets say we had set a full day to 8 hours) to come up with 32 hours?