Help making a desicion on my multyplayer apllication

I'm simply wondering if using log in to Facebook, Google Play or Some Other Means to create player login, and which one is the better route to use, or is to use all 3 such as a redundant backup on a backup? And if so what is the best method to follow to achieve my goal for All platforms and Not just Android devices and iOS devices. Is that even possible for a Computer build or would i require a WebGL version instead, abandoning my Steam idea? I Have a score board I'd like to replace with Google Play's Achievements and Leaderboard's if possible.

I don't know much about multiplayer applications, MySQL PHP or where to start, Also I'd rather go a dedicated server route for Steam, because i was reading somewhere on here that P2P will be no longer supported in 2 years time and a conversion to the new system will be in order.. Is that for all platforms?? I don't wanna redo my work later when it changes, I'd rather set it up to accommodate the change right out of the gate for everything. Luckily I'm still early into the project so figuring this out right away isn't detrimental to me and i don;t see me even getting to a release stage for another year or more that's why i'm asking the question now.

Here is some more details: skim over if you like, main questions are above and below is some background on my build.

What I'm building is a small package of mini games with 15 of them, of various types. Some will be buyable on a store others are free to play with the free download. Free ones are ones that took less then a month to build. ones that took months or years obviously will be paid. I'm not worried about merging all platforms onto one server that i don't even think is possible nor do i want to even try. I'd be making an Android version for PlayStore an iOS version for Apple store and a version for steam for Microsoft and linux because i'd assume its one of the biggest markets for games.. One thing i'm unsure on though, do you need to make a separate build for iOs PC and iOS device? i guess to put one version on steam and one on apple store my answer there is yes i do.

I havn't made a player class yet or any main controllers except my pokemon clone that will not be included in final build for legal reasons, but will be included in a copy for my nephews in which I'll manually install onto their tablets.

i use PlayerPrefs to store my basic data as it does the method i want to do for now simply just to get my code to work. that way, when i have it all working, all's i have to do is change my PlayerPrefs Method Calls to one I create instead. aka PlayerPrefsManager. I have written an encrypting and decrypting Player Prefs Manager but it isn't used throughout all my scripts just yet. because some of the variables i don't want anything to do with player prefs at all. things that are used by it mostly are settings like full screen resolution option and so forth.

Ive built a large part of my core scripts that persists along all scenes, but am holding back on anything to do with actual multiplayer anything until i make a decision what route to take. I've got some basic set up for multiplayer like empty methods for placeholders, but they don't do anything yet.

Anyway, thanks again for anyone who would like to comment