If you don't reckon your skills are good enough to biscuit joint or spline and glue, you could joint the two formply boards with a batten screwed and glued to the underside. A 40mm strip of the other ply would work or use stripwood of similar dimensions.
I have looked into biscuit jointing before and kind of turned my nose up at the cost of the hardware, something like a trim router might be a little limating, but still I can do way more with that then some one off tool.
Dowel joinery or these odd router bits that will dig out channels so that you can press two bits of timber together, was a little more up my ally.
For the rest of it, my suggested design is very much 'bush' carpentry, not fine joinery. The notching in of various parts to the legs is to avoid surface joints loaded in sheer and to avoid notching horizontal beams to retain maximum stiffness. All the notching can be done by hand with a tenon saw, just keep your cuts straight and level and don't over-size the notches!
I'm a little more comfortable with trenching, so far I have used my router and circular saw to cut them out, I have build a set of shelving as a trial and it came out great. The bookshelf also used it too and while it might not be required, still helps me move forward with trying other building techniques
At this level of *rough* carpentry, its probably not worth avoiding screwing into end grain, but do predrill the holes full depth to the root diameter of the screw, then the pies it goes right through to a (min.) clearance hole. Keep all holes at least 10 mm from the edge to avoid splitting, and use long chipboard screws for enough grip.
N.B. due to the clearance hole over the chipboard screw thread this sort of joint is not suitable for significant sheer loading, but screws into end grain shouldn't be sheer loaded anyway. If you need that you'd need to cross-drill for a barrel nut, use a bolt in a hole that's a tight fit on its shank, and ideally notch in the joint to resist splitting forces on the end grain.
I am using this stuff as support,
https://www.bunnings.com.au/42-x-19mm-3-0m-dar-pine-premium_p8401459 something I can jam a screw or six into for support, I used it for the bookcase and display shelving.
As for screws, I think they are deck screws or some kind of outdoor stuff, even then my first set of shelves are using them and for about two years I think, are holding up a bunch of computers and some other hardware.
In terms of joinery, it is best to just stick to wood screws unless you are more experienced with woodworking and have good tools for it.
I had though about using wood glue, but at some point I might need to take them down. So carving out a channel and using screws to hold things together are my go to. I have tried wood glue and love it, for smaller things I have used it.
You can make most of the joints easily by just by overlapping the pieces you want to join and send a screw trough both of them. The clamping force of the screw (or 2 or 3) is usually all you need to hold it together. If it is a joint that might be seeing a lot of torsion stress then you can just put some wood glue in the joint right before screwing it down. That will make a joint that is in most cases so strong that the wood around the joint will fail before the actual joint. It is easy and requires only a drill. Just make sure to get the correct screws with tread in the right place to only grip the bottom piece of wood.
But in general it is best to avoid torsion stress on the joints when possible. Better to add some extra wood into the corners to brace the joint in that direction. Not only does this make it a stronger but also makes it much stiffer (since wood is otherwise a pretty flexible springy material), for workbenches this means it will wobble a lot less side to side. For this particular bench i would just put a back board onto one row of shelves to act as the bracing in the left and right direction, same for the sides to prevent forward/backward sway. Or you can avoid bracing by screwing the back of the bench into a wall, that makes it feel really stiff (but i try to avoid that).
I am not allowed to put holes in the walls, so this had to be free standing.
I do wish I had more money for timber and help with transport, but I don't if I need to I can tack on something else at the back, but I was kind of hoping that the middle shelf would had a bit of strength to the whole setup.
I have yet to mount a LED light underneath the first shelf for better bench illumination,
I'd thought of running an LED light strip along the front of the shelf for illumination. Mine's sized in such a way that there's workspace underneath the shelves, currently I'm using a desk lamp or optionally a magnifying lamp for fine work, but having light that's just there without getting in the way would be nice.
Oh, you should see my LED's!