sometimes devs have several versions because there are tons of devices with different cpu/displays
they are all stored by google on their servers. when installing an app via playstore it will detect/knows which version you need
for the newbs apkpure have their own installer so you dont have to know which version you have to download. thatswhy they offer an apks version, which can be only used with their installer
when scrolling down you can choose the right apk for your device manually, so no need for any installer or whatever.
the way is secure cause all apps are signed and any newer phone will give you a warning when install an app which is not signed correctly. apkpure show you the signature on their website and show you if its the same as on playstore (verified)
for dmss there is only one version, which you can download here
So you know.... you have to unistall the latest rev of dmss before installing earlier version.... apkpure wont even install the earlier version of dmss prior to unistalling a later version.... again, details matter...