Note "preferred seats" are no different from your present seats, only closer to the front --- same width, same pitch, same recline, same cushions, ....

Since the cost extra (until check-in -- when are free) they appear to be an empty area. Once check-in starts, that changes.

Preferred seats are an income source / revenue for UA and tend to leave some aisle seats open for later booking (higher fare customers) -- also they are free for preferred corporate customers and elites.

The blue seat, EconomyPlus are slightly different. They have more leg space / pitch and a bit more recline. Same miserable width. And they don't become free at check-in but are free for elites. GAs may place non-elites in E+ once E- fills up.

Rather or not economy fills is route dependent and hard to tell from seat maps in advance due to Basic Economy and those that do not select seats.