I have used polyurethane a few times. Each time I've tried to be more careful about creating bubbles, but the finish was still riddled with bubbles.
You're putting it on too thick (pun intended), and as well as that you're overworking the surface. Overworking with a brush is the classic way that bubbles are introduced into the varnish film on the surface, although of course some bubbles may be present in the liquid prior to application (for example if the contents are mixed by shaking instead of being stirred).
Just the act of dipping the brush in the can and dragging the brush along the edge of the can, or letting it drip, made bubbles in the can.
The highlighted portion is something that is commonly done but you should strive never to dip a brush directly into the original tin. It is incredibly common to do this these days and I see even pros do it on a regular basis and wince every time because it was considered bad practice not too long ago — this is what paint kettles are for*.
You can sidestep the issue of where you load the brush and solve the problem of bubbles once and for all by modifying your standard poly by diluting it (in a separate container!) changing it in essence to "wiping varnish".
Despite how it's referred to wiping varnish doesn't have to be wiped on but that is a great way of applying varnish, especially to large flat surfaces and table legs.
Essentially complete instructions are provided in a previous Answer:
how to fix very light scratches in polyurethane finish?
(be sure to click on the images to see them at full size so you can read the text more easily)
Note: I shake my homemade wiping varnish in the jar it's made in and this means it is chock full of bubbles right before use, but the thinning and the application method are so effective at eliminating bubbles that there are never any left in the varnish film once it has dried.
*Users aren't solely to blame though, the makers of paint kettles aren't helping matters by having largely retired the square type in favour of round ones, when a straight-sided container is vastly superior to paint out of because it allows the brush to be evenly unloaded.