Many Japanese automotive parts are already produced in Thailand and Indonesia.
That is true. You are technically correct. But those are produced mainly for Japanese car brands themselves (Isuzu/GM mainly in Thailand and Toyota in Indonesia).
But AFAIK Hyundai's parts suppliers are mostly (90%+) located in Korea or are internally sourced and produced in their captive plants in Korea which includes the KIA brand factories. The rest (only a limited number) of the parts are sourced from China. They really squeeze their suppliers on cost but make it up on volume, however lately Korean costs have been going up, so they are looking for suppliers overseas.
HYUNDAI and KIA (at least in the US market) are sold on equivalent common platforms. They learned this trick from GM and FORD (especially GM) where the same car platform underneath the differing outer sheet-metal styles used to be offered in different brands (Buick, Oldsmobile, Pontiac etc.) and offered different marketing images.
The bread-and-butter cash-cow model for Hyundai and Kia in the US (and Canada) is the Hyundai Sonata and Kia Optima, both are equivalent in size, cost and performance to the Toyota Camry, which is a very solid offering in the US family car market. Sonata used to be priced 20% lower than the Camry (Optima even lower) a decade ago. But lately Sonata is not so price-competitive anymore, which is by design by Hyundai/KIA because their quality went up a few notches and is considered roughly equivalent to Toyota.
Some people in the US though still haven't forgotten the early 90's, 2000's bad quality Hyundai products (Hyundai Pony/Excel? anyone?) and they will never buy a Hyundai. A sad thing which Hyundai and KIA are trying to reset in people's minds. Back in the 50's, the first Toyota brought into the US was so weak, it could not climb up some hilly roads. Quality is an uphill battle (literally).
So - back to Hyundai Sonata (Top) and KIA Optima (bottom). See the different styling cues?
Sonata offers more flowing, poetic, classic lines, amenable and appealing to older Americans and females of different age groups. A traditional style.
Optima is a bit more European looking, appealing to younger American males.
Both offer tons more standard features for the money compared to Camry.
Inside, the same styling cue differences continue...Hyundai on top, KIA bottom.
However - the reason why I rambled on this long is to point out, that underneath all these differing styling exercises, the basics, (meaning Car chassis, suspension, wiring design, Engine, transmission, other engine parts) are mostly the same for both Optima and Sonata. Same for other KIA/Hyundai models such as SUVs, MPVs, Trucks etc. Go to any car dealership and check it out. My two cents.