Whoever said PS1 games were not worth it? It can be annoying and emulation is fiddly enough (the most I have to config the average 16 bit or older emulator is my controls of choice, the PS1 stuff might take a bit more, still plenty doable but people like fire and forget) that some don't look so much into it but that only means there is more out there to look at which nobody has managed to do.
It can get a lot more complex but as a start the PS1 used a CD format and copied the standard iso9660 which means you can look at most games with... everything really. Pick an iso manipulation tool and it will probably get you somewhere. Various devs (though often Square Enix) can do things a bit differently but cross that bridge as and when. Problem is there were no scene standards in the PS1 days so if you stick to old scene releases you will have about 10 different formats in heavy rotation and many more outside it. Again though most modern general iso bothering tools can handle most things here.
There is a common graphics format called TIM (the PS2 getting TIM2), many viewers and editors exist.
Being a CD then many games used CD audio, though of course there are other options.
3d is a pain on the PS1 owing to it being so different to most other things before or since, however it is still 3d in the end so meh..
The executable format is based on ELF you might have seen in many other places, the processor is an earlier MIPS device. ARMIPS is a good assembler
https://github.com/Kingcom/armips . I am not sure what we are linking for hardware specifications these days so in the absence of a more informed choice I will go with
http://problemkaputt.de/psx-spx.htm
Everything else is pretty standard ROM hacking.