Hello. Bootloader and baseband software are going to be closed source in your phone. There is no going around them. So you cannot have 100% of the rest of the phone as open source. Then there is also the code in TEE. That would also be closed source. I think if you dig a bit you'll find that the best you can do to run open source code is the kernel and major parts of ROM.
.
You just have to blindly trust that this low level closed source software that you cannot replace would not do anything suspicious, and that it *cannot* be exploited either.
.
You just have to blindly trust that this low level closed source software that you cannot replace would not do anything suspicious, and that it *cannot* be exploited either.