how do rental companies determine which trim lines to buy?
 

I notice it is easy to get high trim lines in American cars. In Japanese cars it depends on the manufacturer. Nissan, yes. Toyota, no. (I don't see enough Mazdas to be able to make a conclusion, nor Subarus).
In German manufacturers like BMW they give you a stripped-down car.

Is it based on how much they get it for (presumably American cars are cheaper), how much the resale value holds, or something else?

Usually its a mix of resale value, manufacturer incentives and overall availability. Certain brands limit how much of each trim gets produced for fleet sales.

Rental companies buy their own fleet and they get leases from the manufacturers. When it comes to purchasing their own fleet, they'll likely buy as close to base as possible to keep the initial cost and depreciation costs down. Most fleets are purchased this way; think of how often you see a company branded car with steel wheels. The manufacturer that provides the leased vehicles determines what trim level to send, given that they'll be receiving them back within a certain period of time/miles. Higher trim cars are typically the first ones produced when a new model comes out, and they're easier to resell as a CPO vehicle, hence why the manufacturer will prefer to provide these on lease.

My experience with National has been:

Ford - Almost always fully loaded or close to it
Chevy - Almost always LT, occasionally Premier
Nissan - Almost always SL or higher
Kia/Hyundai - Always base trim
Chrysler - 300 and Pacifica only, which come well equipped even at base trim
Dodge - Not many but they vary
Jeep - Usually Limited, occasionally base
Toyota - Almost always base
Mazda - Midlevel
Volkswagen - Jettas are base, Atlas are midlevel
GMC - Usually SLT

Surprisingly the recent Toyotas have been the sporty type, SE.

And I guess I've never seen a higher trim Kia/Hyundia. Never knew they existed.

I don't know Toyota trim levels as well as other brands, but I hardly ever see one with leather seats, which is my benchmark for higher trim since trim levels rarely change any of the driving dynamics.

I've never seen a rental Hyundai/Kia with a leather interior except for a few Sedonas. It must be a policy not to sell high-trim cars to rental fleets. But they do exist in the general market and are very nice.

Alaska373890 and oftech pretty well nailed it on the head. I'll add that for every combination of make/model/trim/mileage at resale, the rental car companies know, almost down to the penny, how much profit they'll make off of each vehicle before they even purchase it...

unit cost - depreciation + rental revenue + resale price

Typically they load up on the trim levels that will be the most profitable in the end - less profitable trim levels are usually manufacturer leases that get returned to the manufacturer while the car is still "young" by rental standards.