Hey, nice clean work! Effort is 50% the battle, the other half is getting good reference and information... This is where it helps having lots of varied sources on anatomical reference, and ideally some 3d stuff as well (I personally use sketchfab.com a lot for this, but I also study autopsies and cadavers for cross reference and verification). The deltoideus group (it's a group), inserts at roughly 1/3 to 1/2 from the top of the humerus, on the lateral side (insertion area being about an inch long), very slightly more anterior than posterior. The bottom portion of the insertion is mostly taken up by the posterior head (scapular origin) and medial head (acromial origin) , the anterior head inserts a little higher (proximally) and tucks underneath the medial head. The deltoid thins out a lot before inserting however, and is partially squeezed between the biceps and brachialis and partially has some tendonous tissue connecting to the brachialis. So on the surface appearance, it'll look like it's inserting a little higher than 1/2 and a little closer to that 1/3. You can also google "humerus origins insertions", and it'll show you where each muscle generally goes on that bone, recommend doing this for the scapula and clavicula as well, to get a better idea of those. And as for the deltoid, pertoralis interaction, in that third image specifically. It's a tricky one, they can kinda blend with each other and with less muscular people, the clavicular gap between the pectoralis major and deltoideus anterior origins gets less visible. But the pectoralis major should get more space there, they overlap a little, but not that much. So you'd end up with a 'mostly' straight line between them. You can very roughly, in this pose (not always), draw a straight line between the top edge of the pectoralis major and the top contour biceps (with a dip where pectoralis major inserts, and bulging again at the biceps naturally). The deltoid 'sits' atop the the pectoralis major, but it doesn't droop and if there is little hypertrophy, it doesn't push it away as well (which can make some things very confusing some times, watch out for that!). It would be very close to the second image in form relationships, just from a different view and angle. But the pectoralis major still wants to go to it's anterior spot on the humerus, hugging around the biceps a bit (taking a little detour), but because the arm is rotated a bit, it is pulled up a bit more. So the deltoideus will remain mostly on the medial-anterior side of the arm and the pectoralis major will take up more space being stretched. This is where mostly knowing the direction of the arm is crucial, and then also figuring out the location and position of the clavicle is helpful. So you would first figure out where the clavicle is, then the rotation and angle of the humerus, and then connect all the muscles where they should go. This is also why internalizing the shape and proportions of all the bones in 3d can be super helpful, because you can use any landmarks to figure out the other relationships of bones and muscles in a pose and kinda determine what muscle you're looking at. But also why knowing only 2 muscles is not enough, because the biceps is also interacting here. I don't have my tablet here right now, so you'll have to do with a rough and shitty mouse paintover done in Photoshop, hopefully it clears it up a little though. There's actually a little hint in the reference, that's almost not visible in the image you provided due to compression. But you can see a little curved shadow near the clavicular deltoid origin, this tells you where the divide is.

I´m very happy with this one :) I found it difficult to design the wrinkles of the thumb´s knuckle. If you have any tips, feel free to share.

You're already well on your way! It takes a whole lot of time and practice to develop bold confident lines, that are also precise. And the 'only' way to get there is to draw bold lines as if you are already confident, but slowly, focusing and paying attention to what your body's doing so you can spot the source of mistakes and correct them. Simply put, make a mess, be bold, but don't stop thinking. Finding the right speed is something you gotta discover through trial and error though. Slow down as much as you can, to not be sloppy but not so much that you start overthinking your movements. It's a tricky balance. Also, looking at the pictures, it seems like you're drawing fairly small, possibly from the wrist as well. While it's good you're using a pen and doing some Drawabox.com stuff, make sure to also do the drawing from your shoulder and using bigger strokes part, draw much much bigger stuff, like 3-4 times as big. Just keep truckin' and enjoy the feeeel of the line, this skill can be an endless pursuit as you'll continue to raise the bar for both precision and boldness of strokes.