Definitely not a stupid question!

The UA website showing you each segment individually is a user experience change, not a change in the way fares are constructed. In circumstances where the question "how much does this segment cost?" has a complicated answer, the display price from the United site is often misleading and/or wrong.

What sort of trips are you considering? In most circumstances, a round trip ticket will either be the same as two one-ways, or it will be less (as is the case where special round trip fares are offered). There are also some very rare circumstances where two one-way tickets are less expensive than a round trip, and somewhat more common cases where a multi-segment international trip can be split between tickets to avoid various fare rules (much debate about whether this is a legitimate practice).

But United has not moved to the LCC model of offering all one-way fares.