Think of the snap points more like sphere's, different objects have different snaps available, get them close enough to each other and they snap. The pole for instance has 2 snap points available, one centered on the top face and one centered on the bottom face.

There's a few ways you can try to get the stubborn objects to go in, for foundation poles try looking at the ground where the final piece is going, if the 2m pole's top snap point lines up close to the existing walls snap it'll click in.

It can also be difficult to snap things in from above as you can't really look through a floor or wall, much easier to do it from the bottom but there's ground there, for this reason I usually start builds on levelled ground with framing up on at least 1m poles, If I have trouble snapping additional poles in I can usually press X to sit down and shift the camera just enough to click them in.

Alternative placement just lets you place building objects without snapping to anything, it works more like placing furniture or item stands in that regard.