Shifting direction is different for front and rear shifters I have been riding singlespeed and 1x setups for decades. The last time I had a front derailleur was over 30 years ago, and the shifter was on the downtube then, and I don’t have any memory of how it all worked. So I’m experiencing a time warp of sorts when it comes to multi-geared bikes.
Recently I bought a cargo bike (Mongoose Envoy) with a Shimano 3X drivetrain. I love having the extra gears, but I’m totally shocked that using the thumb lever on the right shifter gives a downshift, but using the thumb lever on the left shifter gives an upshift. And vice-versa for the triggers…using the trigger on the right shifter gives an upshift, but using the trigger on the left gives a downshift. It’s completely counter-intuitive to my brain and I can’t imagine the reason it would be this way.
First of all, is my bike assembled right? Is this the normal way all bikes are? If this is the “standard” setup, how did this convention ever arise in cycling? Is there some advantage to the controls being opposite that I’m not able to appreciate? And lastly, can I change the cable routing somehow so that the front derailleur both shift the same “direction”?
The reason why shifting is arranged this way:

Shifting to a larger chainring at the front gives a higher gear ratio, shifting to a larger sprocket at the rear gives a lower gear ratio.

Shifting to larger chainring/sprocket requires positive pulling force from the shifter whereas shifting to smaller chainring/sprocket can be done with derailleur spring tension. It's intuitive that getting the chain to  climb up onto a larger chainring/sprocket will takes more force than letting it fall down.

Its easier to get positive pulling force from the thumb shifter lever then the finger lever, there's more room for a longer lever and a longer lever throw. The thumb can travel on longer distance without the  hand losing grip.


The exception is of course Shimano's Rapid Rise rear derailleur which use derailleur spring tension to shift to larger sprockets. THis obviously didn't work as well as 'normal' derailleurs as the type has disappeared.