The Exynos versions of the Samsung Galaxy S9 and Galaxy S9 Plus have received TWRP support which means individuals with those models will be one step closer to loading on custom firmware. TWRP for the uninitiated is a custom recovery option which is basically a custom bootloader that gives you more options, something you'll need if you plan to root your phone and flash any ROMs.
There are multiple ways to install TWRP for each phone, though the easiest method in terms of just installing TWRP, requires your phone to already be rooted. This is the app install method and the app itself can be download from the Play Store but it will require root for it to function. If you don't have a rooted device then you can use the Odin install method so long as you have Odin installed on your PC.
Each version of TWRP regardless of whether it's for the Samsung Galaxy S9 or Galaxy S9 Plus weighs in at about 44MB so it's not an incredibly large file, and that means you should be able to download it rather quickly even if you're doing so over a cellular connection as 44MB won't eat up too much data. Keeping all of this in mind it's worth remembering that since these downloads are for the Exynos versions of the Galaxy S9 and Galaxy S9 Plus, users in the U.S. who have purchased either phone won't be able to utilize TWRP yet as the U.S. model of both phones runs on the Qualcomm Snapdragon 845 processor, and not Samsung's in-house Exynos CPU. Unfortunately for U.S. device owners that means that installing any custom firmware won't be possible at this point in time, and there's no telling when TWRP will show up for the Snapdragon model, though the device did just officially launch today and it may not be too long as Team Win tends to be quick with updating their device support.