They are two different lines. The 3xxx series is entry level. The 5xxx series has a few more features. To see what the differences are, compare the specs on the specific two you are interested in.
I googled it for you. I’m sure there are several similar comparisons online. This was the first one to come up.
Nikon D3500 vs Nikon D5600 Detailed Comparison
https://cameradecision.com/compare/Nikon-D3500-vs-Nikon-D5600
Nikon D3500 Key Specs
- Announcement Date: 2018-08-29
- 24MP - APS-C CMOS Sensor
- No Anti-aliasing (AA) filter
- ISO 100 - 25600
- Nikon F Mount
- 3" Fixed Type Screen
- Optical (pentamirror) viewfinder
- 5.0 fps continuous shooting
- 1920 x 1080 video resolution
- 365g. 124 x 97 x 70 mm
- Replaced Nikon D3400 Compare
Nikon D5600 Key Specs
- Announcement Date: 2016-11-10
- 24MP - APS-C CMOS Sensor
- No Anti-aliasing (AA) filter
- ISO 100 - 25600
- Nikon F Mount
- 3.2" Fully Articulated Screen
- Optical (pentamirror) viewfinder
- 5.0 fps continuous shooting
- 1920 x 1080 video resolution
- Built-in Wireless
- 465g. 124 x 97 x 70 mm
- Replaced Nikon D5500 Compare
Reasons to choose Nikon D3500 over Nikon D5600
2 advantages
Battery Life 1550 shotsvs820 shots 730 more frames with a single charge
Weight 365 gvs465 g 100 g lighter
Reasons to choose Nikon D5600 over Nikon D3500
11 advantages
Wireless Connection Built-InvsNo Better connectivity
Articulating Screen YesvsNo Flexible shooting positions
Touch Screen YesvsNo Easy control of camera functions
Number of Focus Points 39vs11 28 more focus points
LCD Screen Size 3.2"vs3" 0.2 inches larger display
LCD Screen Resolution 1.037k dotsvs921k dots 12% higher resolution screen
Microphone Port YesvsNo High quality audio recording option
Timelapse Recording YesvsNo creative shooting
Flash Coverage 12.0mvs7.0m 5m longer range
AE Bracketing YesvsNo Useful for tough lighting conditions and HDR
NFC Connection YesvsNo Easy wireless connectivity with compatible devices