If i own a module hard copy in real life, do i have to pay for it on roll 20?

That's it, that's all i really want to ask

To get access to the module in Roll20, yes you would have to pay for it again through Roll20.  Roll20 doesn't own the rights to any of the content - they simply license it from Wizards of the Coast.  The same is true for D&D Beyond, or any other VTT.

Bear in mind, the module in roll20 is genuinely a different product than the hard copy. It includes all the material in the hard copy, but also  includes stuff made to work for roll20 - handouts formatted for players, characters and monsters with tokens and macros already made, prepared encounter maps, with dynamic lighting already set up, and so on. If the module is set up well, it will be a far superior product than the hardcopy, when trying to use it on roll20. I havent bought any modules, but from comments I gather the quality is variable. Some are really well done, and others are less so. If you're planning to get any, I'd look for reviews first - which is good advice for any product.

I like to think of this as "owning the novel doesn't let you see the movie for free."

They are two different medias by two different companies. The Roll20 module is a time saver in terms of prep. Do you need to purchase it if you have the hard copy already? No. You can upload the images yourself (pay attention to their sizes due the upload limitation and quota limit). You can create all the various characters yourself (take a blank sheet and fill it out, for every monster and npc), Prep all your tokens (attach the needed sheet to the token then save the sheet so the token is now the default - pay attention to the mook aspect for monsters), Place all your needed monsters on the maps, Create any and all handouts you are going to use, Create any card deck you are going to use, and the list goes on but you get the point. All that work is done for you with a module you purchased. This saves a person a ton of time and work. I hope this explains things some for you.