A simple way to turn standing yoga poses into shoulders strengthening exercises is to shift weight and press your hand or hands into the floor. (Another option is to use your arm muscles against each other to both strengthen and stretch your shoulders).
One option for standing shoulder strengthening exercises is wide leg forward bend. Start with your elbows straight and hands on the floor ahead of your feet. Shift your weight onto your hands and with elbows straight press your hands strongly into the floor. Then shift back and relax. Repeat a few times.
If you are more flexible then bend your elbows and position your hands between your feet shoulder width apart. Here, because your hands are in line with your feet you won't be able to shift your weight to your hands. Instead focus on pressing them strongly into the floor without straightening your elbows. You may find that it feels as if your feet get lighter on the floor, as if you are about to lift into a bent elbow handstand.
Speaking of handstand, downward dog is another pose that you can use as a shoulder strengthening exercise. You can then use l-shaped handstand with the same intent.
In triangle twist you can do something similiar. Start with your hand lightly on the floor (use yoga blocks or a chair if you can't reach). Shift weight towards your hand and pressing down into the floor. Relax the hand (shift weight back to feet) and then repeat.
You can do a similiar action in half moon pose. Start by in half moon stand with your weight over your leg so that your hand only lightly touches the floor. Then shift your weight towards your hand. As you do so press your hand strongly into the floor.
Then shift your weight back to your foot and relax your hand.
A slightly different exercise is to simply focus on pressing your hand into the floor. The weight shift may happen automatically based on the idea of strongly pressing your hand into the floor. You need your body weight shifted towards your hand in order to press strongly into the floor.
If you can touch your hand to the floor in side angle pose (again, use a yoga block if you can't) try pressing your hand strongly into the floor. Imagine you wanted to lift your front leg. Relax and repeat.
You could use side plank as a shoulder strengthening exercise in a similiar way.
In a standing forward bend with your hands on the floor (you can bend your knees if you can't reach, or place your hands on the edge of a chair or elbows on the edge of a counter top or table), shift your weight forwards so that your hands bear some of your body weight. Then deliberately push your hands into the floor, strongly.
An initial approach to this exercise is to shift forwards and push, then shift back so that your arms relax.
Another option is a bit more subtle and requires some body awareness.
With your hands on the floor in a standing forward bend, shift your weight to your hands and keep it there. Keep your pelvis stationary and lift one leg. You'll have to brace the standing leg and the opposite arm in order to keep your pelvis stationary. In half moon pose you can also weight shift to strengthen one arm at a time.
One other option, if you are flexible enough, is to place your hands besides your feet. Then instead of shifting forwards and then pressing down, just focus on pressing your hands down strongly.
And that is the key point, focus on pressing your hand down strongly. As you do so you should feel your arm, and your shoulder activate. In the case of standing forward bend, you may feel your feet getting lighter on the floor.
The Frictional Arm and Leg Strength video includes a sequence of yoga poses designed to teach you how to use both pressure and friction to strengthen your shoulders and hips.