The overlays are like the raspberry pi overlays and all additional and configuration options to be added without need to convert and recompile dts/dtbo.
https://wiki.radxa.com/Rockpi4/hardware/devtree_overlays
5.2 / 5.1 is mainline and uses the mainline device tree whilst rockpi4-dtbo is specifically for 4.4 that contains many backports.
5.2 RC5 has been released but there are some quite major gneral changes to memory availability and case agnostic Ext4 that might need patches, might get shifted to 5.3 but those are pretty major linux kernel changes and not just rockpi4 specific.
Unfortunately the patches for Wifi/Bt and PCIe are in 5.3 and not particularly large but would need to be patched to 5.2 to get them running.
@ Antram made a great summary