[Samsung Galaxy S9  Questions & Answers] jag9906:
A bootloader is a partition that tells the system when powered on which environment to load. All systems computers phones etc have a boot of some type each different in protecting or the lengths they go to insure we can't alter it to stop people from F'n around with it, unlike those who release source code like GNU or similar BSD etc communities/lic. which open options easier and gives us a way to delve into dev/hack/alter /what ever you want to call it and easier understanding of the processes involved. There are some bright people and others working and gaining knowledge within there workplace who help the understanding here for these locked bootloader and systems and then share. Which is admire when dealing with manufactures like samsung who think you should not be able to alter product you straight up own. Thats when the locked bootloader steps it up, and at least for awhile they control and still make all the decisions on the devices we buy but it's odd other geographic locations have unlocked loaders I dont know for sure of course but could very well be a request from government like NSA. With locking it down and tools like Knox they know every detail of your life if that phone is glued in your hand. So installing a custom recovery which is usually your first step in updating your phone to install custom roms, root, mods kernels etc dumping things like bloat, samsung spyware, streamline, etc and really making your own choices and also the phone becomes a reflection of someones tastes attitude etc.

BUT A locked bootloader is not a 100% lock down of a system, or at least in recent past. But many things I am still researching can play into it. I was able to install custom recovery with a locked bootloader in the past, and with qualcomm mode I have read that you can or use to be able to install things such as recoveries etc. Now this might be limited to older versions of Android I am not sure, unless you replace boot or if there is a back up recovery/boot that will just get copies back over your custom img. So needing to copy over primary and backup then moving on from recovery to root, kernels, ROMs, Mods etc. The backups also can fix corruptions in the primary during boot not sure unauthorized img. Now I just recently got my hands on some old Snapdragons I am going to check this and I am unsure if its limited to older android versions like I said.

With the device I had for a short time which had a locked bootloader I ported a twrp recovery from another device and I was able to gain a temp root, and using flashify to install the twrp img withing the android OS and temp root on the phone. I then jump into recovery installed root and make it perm. With these snapdragons I have some samsungs s4. s7 possibly s8 and recent motorola's moto e4. z play and few more. I am going to see about going this route I know someone willing if I can get the s7/s8 recovery or boot to stick thats will try on his s9 plus. I am willing to trash these other devices to explore, learn and give samsung the finger. So by the end of the month I hope to at least see/learn if any of these will work with that mode, cause the trick i used with temp root probably will not happen.