That is a good question. FCP is supposed to give you the option, when creating a new library and project, of whether to store your files within the library or leave them in their external location. The software seems to have a mind of its own in this regard, though, and I am not enough of an expert to explain why. Sometimes the option exists, other times it does not. Within "Properties" (File > Library Properties) you have some options that can be changed, which allow some storage modifications, but I have not found it particularly helpful. That is due to my lack of knowledge, not due to the program. If, when creating a new library, I see the option to "leave external files where they are," I choose that. If I don't see it, and FCP insists upon copying the originals to the library, I just put up with the large library size until the project is finished. Then, making an archive as I mentioned will allow me to delete the original huge library.
You are very wise in paying attention to the organization of your workflow and files before beginning. It can get out of control quickly. For example, when I first started using FCP, I would edit footage I wanted to use down to shorter clips and save them with different names. I thought this would make editing easier. I would then import them (File > Import > Media) and work on my video. But, I realized I would have to save those edited clips too, if I ever wanted to work on that file in the future, which consumed even more storage space on top of the original footage! This is especially bothersome with 4k or larger files. So now, I just import all the original footage necessary for the project, and I rarely worry about any pre-editing.
The only time I will pre-edit is if I know I have a lot of useless stuff within a given clip. Then I'll do a rough cut to eliminate the junk, and save the edit as my "original," dumping the unedited original out-of-camera footage. The lesson here is to not let video files proliferate unnecssarily, since they just require massive amounts of storage space and become confusing over time if you need to relink an archived library to them.
You are very wise in paying attention to the organization of your workflow and files before beginning. It can get out of control quickly. For example, when I first started using FCP, I would edit footage I wanted to use down to shorter clips and save them with different names. I thought this would make editing easier. I would then import them (File > Import > Media) and work on my video. But, I realized I would have to save those edited clips too, if I ever wanted to work on that file in the future, which consumed even more storage space on top of the original footage! This is especially bothersome with 4k or larger files. So now, I just import all the original footage necessary for the project, and I rarely worry about any pre-editing.
The only time I will pre-edit is if I know I have a lot of useless stuff within a given clip. Then I'll do a rough cut to eliminate the junk, and save the edit as my "original," dumping the unedited original out-of-camera footage. The lesson here is to not let video files proliferate unnecssarily, since they just require massive amounts of storage space and become confusing over time if you need to relink an archived library to them.