Are there 2 ways to create fields for a module: Studio, Module Builder, right? Could u tell me which options( Studio or Module Builder ) is better and best practice to create new fields for this existing module in SuiteCRM.
Module Builder is used to create new modules, fields its relationship with another module etc. You can then either Deploy it and keep on changing there itself. Or Publish it and then install it.
Whereas Studio is used to make changes to the existing deployed(Installed) modules.
In your case, you can do it from the studio. Also, you want to create fields in default modules then Studio is the only option.