lemme just shotgun some advice I was given with F360... the best way to learn is to keep asking (plenty of videos. plenty of help on the F360 forums, too,)

First, is to create a master component. Especially for more complicated parts. So, first thing you do is create a component, this top level is the whole thing you're making. For your box example, this would be both the box and the lid itself, both set as sub components to the master. I would also consider importing a .step file... which you can then reference. instead of reverse engineering your Uno. (well, double check and make sure it's accurate, most will be.) you can also import .stls, but there's a few more steps there- you'd have to slice the mesh and then define what all the features are.

Second is try to use master sketches- but don't try too hard to get all the odds and ends into one profile. if it starts getting too cluttered, hard to follow and such, time for a fresh sketch. If you were going to make something that has a few differing profiles, all extruded on top of each other, you can use one sketch for that entire process, and change where you start the extrusion from in the extrusion menu. Adding onto this, starting and finishing "at object" will let you start or finish an extrusion at a face or body. This is great if you want to punch oddly shaped holes into a wall, but not opposite wall. or pillars to connect things.

Then... learn to love parametric designs. There's a ton of videos on how to do that with f360, but under the 'modify' drop down menu, there's the 'change parameters' thing. that opens up a pop up menu to see every dimension you create in a given file. From there, you can also see the ID tags of every dimension in the design. if you created a few in a sketch and want to reuse them across multiple other places; you can. Just hit the "favorite" star and name it something easy to follow. Or you can create your own to start with. just plug in the name you give it or the tag created by f360 (d3, d45).

You can also do some fairly complex math (including trig, bleh.), but for example, if you want the get the Uno to float 1/3 above the bottom, you can define the box's depth, reference the boxes depth as multiplied by 1/3, to create the lower stand offs, and then on the lid, reference the depth again, multiplying it by 2/3's. And then you print it and discover you forgot to account for the thickness of the arduino, so you go back and subtract half the thickness of board from each side. (something we've all done. this is part of why I suggested importing- or creating- a rough stand-in for external parts. it lets you sanity check things before you find out you forgot something)

Finally... have patience. F360 is incredibly useful and powerful. It's great. but it has steep learning curve.