Creating a seamless open world without loading beams is not an easy task. There are many small and large problems that need to be solved. This is not a beginner project!
The basic approach is usually to divide the world into sections and load only the sections that are around the player. When the player moves around the world, new sections and their assets are loaded from the hard disk into memory and sections further away are deleted.
If you want great visibility where you have to load many, many sections, you usually load the more distant sections with a lower level of detail. Simplified geometry, lower resolution textures and no simulation of moving objects. As the player gets closer, these low detail sections are replaced by high detail sections. This requires that you have multiple levels of detail for each section.
The radius in which you load sections with full, medium, and low details is a matter of memory budgeting. You should determine early on how much RAM is expected from end-user devices and how much RAM you can budget for the environment. Do a back-of-the-envelope calculation of how much RAM a section needs at each level of detail. After you have implemented the first sections, confirm these assumptions by creating a profile. This should tell you how many sections you can afford to load at the same time.
If you are targeting the PC or phone market, the functions of end-user devices can differ significantly. So you can specify that the quality level ranges are configurable for the end user. You can also try to determine the user's memory and dynamically adjust the view areas.
It may be a good idea to move this constant loading, replacing, and unloading of sections to a separate thread so that the main game doesn't slow down. But now you have to deal with what happens when the player moves very quickly and manages to enter a section that is not yet fully loaded. You could put the player in a low-detail environment, but this will result in a very poor gaming experience and groundbreaking mistakes. Displaying a loading bar may be the better solution to this situation.
