What does it mean for a traveler to be "from a country" for the purposes of travel to Spain?

Normally origin means departure.

In practice, it means the government is 1) allowing airlines to operate flights from the listed countries, 2) allowing passenger ships to enter their harbours 2) reopen its land border with France and Portugal.

First, remember that Schengen travels, under normal rules, are domestic at all effect, so no border stop, no systematic inspection, no questioning. If you fly from Finland to Spain, your "origin" is Finland and it is very unlikely to get questions under normal rules.

That said... For the purpose of health controls and disease prevention, it may be also important your RECENT travel history. Once at the external Schengen border, where people are questioned, authorities may ask the traveler for travel history and in particular inspect passports.

In a few words... Your passport/residency is meaningless in the context of disease prevention

• If you can fly Brazil to Span via a third country, your origin is the third country, unless you are questioned...
• Once you enter the Schengen Area, police border officers can ask about your last visits, and you will be legally obligated to disclose you are coming from Brazil. Not sure if you can be really denied entry (certainly not if you are EU/EAA), but if you have been to Brazil in the last 14 days, it is likely that you will be subject to quarantine and/or screening.