I am saving files to USB and also to hard drive but despite being in LibreOffice.calc the computer seems to want to hijack me to Excel. How to save files without this? I do not want to access on-line storage.
Can I use autosave?
ODF means Open Document Format. This is the native format of files used by default by LibreOffice (as well as Apache OpenOffice and some other applications). You should keep your work in this format and only after your have finished export to other formats as necessary.
If you need further help, read these guidelines and ask a good question. In particular, you write:
computer seems to want to hijack me to Excel
First, computers do not want anything, as they are not living beings. They just do what they are programmed to. Second, what is hijack me to Excel?
Just for a guess, the questioner is using Windows, and a recent Windows update has set the file associations for ODF files to be MS Office instead of LO.
This is a good guess. Very probable. But until we get more meaningful details from the asker, it is just a guess 
There’s no harm in suggesting what may be happening and offering a solution on that assumption. It might be more helpful and could save time.
There is harm actually. Posting answers based on assumptions means in most cases creating useless noise instead of providing useful information. The purpose of this site (and similar ones) is to be a knowledge base, not an assumption base.
Thank you for your correction of my approach. I had thought that a comment would be most appropriate for a suggestion of this nature rather than an answer but apparently not.
I shall refrain from any further participation on this site lest I further transgress.