Winds
FMC's use current conditions for some ways out in front, then start averaging between actual conditions and forecast conditions as put in by you until a certain range, beyond which they will use exclusively your forecast. Actual changeover ranges may depend on aircraft type (slow, short range, or fast long range aircraft) and on FMC manufacturer.
For descents, there will be an algorithm, based on the "standard phenomenon" that wind speed increases with altitude (conversely, decreases during descent), which may, for instance, make a straight interpolation between a value of 0 kts 2.000 ft below sea level and xx kts at a level for which a known wind exists (either measured or forecast).
For wind direction, again straight interpolation, meaning: if between two levels, for which a wind value is known (actual or forecast), there is a difference of say 30 degrees in wind direction, then for a point for which the level will be halfway between those two levels, the wind direction change will be 15 degrees.
If you want to see all this magic in action, just take a look at your legs page (route data, or next page, whatever applies to your aircraft type). Usually, manually input values are in large font, values calculated through interpolation or propagation from other waypoints is usually in small font.