Formula/mutli-dependent drop down help

I am trying to create a multi (6 drop downs) excel workbook with the main dropdown in cell b16, cell c16 is dependent on b16, d16 is dependent on b16, and is supposed to assign the correct category code of 100-199 not to be repeated (not sure how to do this either) and finally the brand category which is cell e16 and this also is dependent on b16. I am not very good at excel and have been trying to find an answer for almost a month now, everyone keeps directing me to contextures, Microsoft and other websites, but these do not have the answers that I need or the help. I have all the information for each category on separate tabs. Please help! I have been looking online for answers for almost a month, an have been to contextures, Microsoft, etc.

it says my file is too large to upload, I have all of my data on tabs because I don't know how else to do it without having to redo the whole thing and it is due in class on Monday.

Screenshot 2022-11-26 at 3.31.38 PM.png
I hope that this screenshot at least will show up