How to identify and remove odd files on MacBook Air?

Is this a new computer or pre-owned?

...odd files with many numbers and letters appear

Adware commonly uses long strings of scrambled characters to make itself harder to remove. We cannot know for sure without seeing the file names. You can take a screenshot of a portion of your screen showing the filenames:

Take screenshots or screen recordings on Mac - Apple Support

using, under the section, "Take pictures using keyboard shortcuts," the option "Capture a portion of the screen."

To post a screenshot here, use the Image Insertion tool in the forum text-entry window toolbar:

Posted images must be in jpg or png format and 5MB or less in size.

You can also post a system config report. Fortunately there is a secure way to do that.

If you want a data-driven evaluation in this setting where we can neither see nor touch your computer, please post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues and interferences including adware, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information but will help us give you a path on which to proceed,

Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

How to use the Add Text Feature When Post… - Apple Community

Please post the entire report. What seems insignificant to a new Etrecheck user can hold answers for those of us who have reviewed thousands of those reports.