Was recently trying to install mods in mc 1.0.0 and 1.1 to try out classic mods but after editing (for example 1.0.jar) ill try to open the game through the launcher and it will redownload the original file (1.0.jar) deleting the mods that i installed... is there anyway to bypass this?
You don't need to use a 3rd party launcher, as my own mods prove - I've only ever used the official launcher for 7+ years with no problems whatsoever (you've had to do the following to manually install mods for nearly as long, ever since the launcher was updated for 1.6, before then the launcher only updated the game when you told it to):
Installation instructions (also included in a Readme in the download; replace TMCW with the appropriate version; it must be the same as the name of the json file):
To install, create a new profile named TMCW with 1.6.4 as the version and click Play to download it, then go to the versions folder in .minecraft (%appdata% on Windows) and copy the 1.6.4 folder and rename it and the jar inside to TMCW. Replace the json file with the one found in the root of this archive; it has already been modified so the launcher will not try to redownload the jar. Install the mod files directly into the jar using a zip utility (do not try to rename it to zip and use Windows because there is a class file in the vanilla jar with a reserved Windows filename which will break things), and delete META-INF.
As for modifying the json file yourself (I include a pre-modified file so all you need to do is drop it in), you only need to edit two things, first, search for "client.jar" and delete the entire "downloads" section that encloses it, and just after that is something that says "id": "1.0.0" or the like - change "1.0.0" to the name you gave to the folder and files.
Here is an image that shows what I'm referring to; everything shown except for the last line is to be deleted with the ID changed to the same name as the files, and no other part of the file touched:
("downloads" was between "legacy" and "id")
Here is the launcher log, it does complain that it can't get the hash or download the file but it starts the game:
[Info: 2020-07-24 18:39:13.7606092: GameCallbacks.cpp(158)] Launcher/launcher (main) Warn Couldn't fetch SHA1 checksum for https://s3.amazonaws.com/Minecraft.Download/versions/TMCWv4.5/TMCWv4.5.jar
[Info: 2020-07-24 18:39:13.7611554: GameCallbacks.cpp(158)] Launcher/launcher (main) Warn Have local file C:\Users\\AppData\Roaming\.minecraft\versions\TMCWv4.5\TMCWv4.5.jar but don't know what size or hash it should be. Have to assume it's good.
[Info: 2020-07-24 18:39:13.7716203: NetQueue.cpp(557)] NetQueue: Action failed in finalization: https://s3.amazonaws.com/Minecraft.Download/versions/TMCWv4.5/TMCWv4.5.jar.sha1
[Info: 2020-07-24 18:39:13.9155541: GameCallbacks.cpp(158)] Launcher/launcher (main) Info Checking installations.
[Info: 2020-07-24 18:39:13.9157690: GameCallbacks.cpp(158)] Launcher/launcher (main) Info Minecraft client TMCWv4.5 is ready to start.
[Info: 2020-07-24 18:39:13.9157996: GameController.cpp(450)] Starting game with token 1