Hi, im currently working for a company that has some inventory control problems
Okay, so since the visuals are done in excel, I think you can start with simple linear regression in excel. Im not an excel expert in this regard, but I think this article will be a good starting point
https://www.ablebits.com/office-addins-blog/2018/08/01/linear-regression-analysis-excel/
So, what you can do:

Get your data and fit linear regression on it.
You need to choose the dependent variable y (this you want to predict) - this will be your inventory variable. All other features (basically other data in your disposal) will act as independent Xi variables.
You fit linear model of form y = bx + a (simplified)
After you have fit the model, you can access its coefficients. One of them will be an Intercept other ones will be coefficients for your input X features.
This coefficient will be X affect on your y (inventory). So, for example, if coefficient is 0.1 for X1, then increasing X1 for 1 will lead to 0.1 increase in y.
At this point you have dependency between input data and inventory change.

Thats a lot more to consider - how good your model is (for this you use metrics, for example MAE and test your model on unseen data), do you have a data-leak, and so on and so on.  You will also need to pre-process your data in viable format, for example extract day/week number from date to try capturing seasonality. Also you should avoid correlation between variable, thats really a lot to think of.
But most importantly, I think you should try to implement it even if you dont know all details and all the rules just to get a fealing if this task fits you, after it you can go on and lear more about all this stuff.
(I would definitely recommend going through this series one day if you are interested https://www.youtube.com/playlist?list=PLOg0ngHtcqbPTlZzRHA2ocQZqB1D_qZ5V )
Hope this helps.