Do you mean the menu bar?
To turn menu bar on you can click that little arrow pointing downwards at the top of your screen (to the left of where it sais "Autodesk AutoCAD 2020") then show menu bar.
The default for dimensions is in the annotate palette group.
Personally, I like to use the menu bar for dimensions because it seems easier to select exactly what I want. But just me.
I use "drafting and annotation' workspace (customized for me) with the menu bar on.
You can change the workspace setting by clicking the gear at bottom right.
Once you get everything set up the way you want you can click the gear again and save your workspace settings.
If you still have access to your old computer one option is to go into tools, options, profiles tab, export. You can then place the file on your company server or email it to yourself. Then import your profile in Autocad on your new machine. It usually works fine but have seen some mixed results across different versions of Autocad.
I really liked the old classic setup but they got rid of that back in like 2016 or something. (TEAR)
-ChriS