The thing about being open is that, although anyone CAN check everything by themself, it’s often not feasible. There is a greater likelyhood that some have checked some of things (possibly many someones for redundancy) - a community effort - and that you can verify your copy is identical to those [I’m making a difference between checking how things work and verifying if things are the same as the source, as generalization is to think them to be the same].
I have no idea about your Librem 14 question - you jump to different hardware from Librem 5. You should cut that and make a separate thread question or use the original announcement thread: How to Hardware Flash your Librem 14 Boot Firmware.