They are simply triangles, or rotated parts, to rotate the brick to seem slanted it must be at a good angle to create any effect. I would just recommend inserting a triangle into studio and playing with the triangle and other parts to try and get the effect your after. Good luck!
Rotating parts manually or placing wedges can be a time consuming process. A better method of creating the single flat angles would be gapfilling between edges. If you want a different angle, all you have to do is adjust the length or width of your two starting parts.
For angles consisting of multiple segmented parts on the other hand, as shown in the second example you provided, you should use Archimedes 2: Archimedes (v3.1.7) - Roblox
Start off with a single brick, then adjust the angle and face using archimedes. You can then rotate using a center point as your pivot to duplicate and wrap the arch around a full circle. This method is used to create the circular base and the arch in the screenshot below.