To remove moisture naturally or by direct intervention
When to use
natural laundry drying, drying towel on head, natural fruit drying
The nuance
Use 말리다 when drying by natural means like sunlight or wind, or by direct human help.
말리다 (mal-ri-da, malrida) · 건조시키다 (geon-jo-si-ki-da, geonjosikida)
말리다 generally means drying naturally or by direct intervention, while 건조시키다 refers mainly to artificial drying using machines or devices.

One lane per word — no nested boxes, just the gist.
To remove moisture naturally or by direct intervention
When to use
natural laundry drying, drying towel on head, natural fruit drying
The nuance
Use 말리다 when drying by natural means like sunlight or wind, or by direct human help.
To remove moisture artificially using machines or devices
When to use
using dryer in washing machine, food drying in oven, laboratory sample drying
The nuance
Use 건조시키다 when drying with mechanical devices like dryers or ovens.
빨래를 밖에 말려야 해.
ppal-rae-reul bakk-e mal-ryeo-ya hae., ppal rae reul bakk e mal ryeo ya hae.
말리다 fits drying laundry naturally outdoors with wind and sun.
빨래를 건조기에 말려야 해.
ppal-rae-reul geon-jo-gi-e mal-ryeo-ya hae., ppal rae reul geon jo gi e mal ryeo ya hae.
When using a dryer, 건조시키다 is correct; '말리다' with dryer is unnatural.
머리카락을 수건으로 말렸다.
meo-ri-ka-rak-eul su-geon-eu-ro mal-ryeoss-da., meo ri ka rak eul su geon eu ro mal ryeoss da.
Drying hair with a towel is a typical case for 말리다 involving direct help.
머리카락을 드라이어로 건조시켰다.
meo-ri-ka-rak-eul deu-ra-i-eo-ro geon-jo-si-kyeoss-da., meo ri ka rak eul deu ra i eo ro geon jo si kyeoss da.
Using a device like a hair dryer calls for 건조시키다.
과일을 햇볕에 건조시켰다.
gwa-il-eul haes-byeot-e geon-jo-si-kyeoss-da., gwa il eul haes byeot e geon jo si kyeoss da.
Natural sun drying uses 말리다, not 건조시키다.
음식을 오븐으로 건조시켰다.
eum-sik-eul o-beun-eu-ro geon-jo-si-kyeoss-da., eum sik eul o beun eu ro geon jo si kyeoss da.
Using an oven, a mechanical device, requires 건조시키다.
수건을 밖에 건조시켰다.
su-geon-eul bakk-e geon-jo-si-kyeoss-da., su geon eul bakk e geon jo si kyeoss da.
For natural drying outdoors, 말리다 is appropriate, not 건조시키다.
세탁물을 건조기에 건조시켰다.
se-tak-mul-eul geon-jo-gi-e geon-jo-si-kyeoss-da., se tak mul eul geon jo gi e geon jo si kyeoss da.
Using a dryer machine requires 건조시키다.
Which expression indicates natural drying?
Dry hair using a hair dryer.
Dry fruit in the sunlight.