Most ROM hacking is similar enough between systems that learning the basics of anything will do. Fonts, tables of text encoding (games often custom encode stuff), pointers, markup done by the games, memory areas being used to store data about all aspects of the game engine, the idea that instructions run on a ... all much the same whether you are on a commodore 64 or PS4 -- the instructions might change a bit (for the most part you are still going to have a few core concepts doing most of the heavy lifting that you will care about until you get really into the weeds) and different memory types are favoured (wasting bytes of memory is a bad plan on the C64, you have some 8 gigabytes on the PS4 so can happily give 64 kilobytes to each stat on a 500 character game and not notice).
The PS1 has the added bonus (and sometimes downside) of being of the filesystem/optical media based persuasion.
This means files are arranged in an iso (for the most part*) and they will usually be catapulted into memory to do anything (even back in the PS1 the 2x aka 300kilobytes a second read speed would have been incredibly slow to try reading in real time for many things). Compare this to most commercial games released on ROM based devices before the DS where it was all packed into one file without easily apparent rhyme or reason and you get to pick apart and find room in it (instead you do get to worry about memory limits). File system also means file names, file sizes, directory names, usually helper files and markers to various include or exclude files from your search for what you want. Being on a CD means you usually (though technically potentially as "have space, will fill it" applies to devs just as much as personal hard drives or shelf space) have hundreds of megs of free space to put your changes in where on a ROM on a 16 bit or older system you might have to fight to get more than 10 bytes in a row to play with at times for anything extra.
*the PS1 was a long time ago and we did not see standards arise and be adopted. To that end there are 1000 different competing formats during the era (plain old iso, bin+cue, nero's NRG, clonecd and a bunch of others, there are tools that reckon they handle them all). It also troubles things when you come to releasing a patch -- do you release a patch for the scene iso knowing people will have their own discs/rips, or go file level and make it harder. Additionally some games, most notably square enix games, would stick files outside this and force raw reads of the sectors involved.