Build a "skeleton" of the tree with all the curves and twists, then thicken it to the right size.

Add roots at the bottom to slightly expand the base of the tree. Roots are often more above ground toward the opposite side the tree is leaning

Add fences and fence gates where smaller branches for more detail. Use stairs and slabs for rounding to make the tree look more natural.

Add leaves loosely connected to you branches with larger clumps around larger branches.

Leaves on Oak trees are generally fluffy round shapes. I used a noise brush with Axiom

You can use several different types of leaves in the same tree. Here I used Oak and Jungle leaves.

Spruce trees are straight and tall so no need for anything too crazy. The trunk of the tree should be a cone shape, gradually getting thinner the higher up.

Since Spruce trees grow straight up, roots wont be very visible. Smooth with stairs, slabs, and fences. Add branches using; more wood, fences, and fence gates sticking out of the base, getting shorter the thinner the trunk gets.

Start from the top with only a one block layer and gradually add more leaves per lever. Leave(s) space between each layer of leaves.

This is an example of a thin spruce tree.

Birch trees can have structure similar to Spruce with a thin trunk, but birch can vary alot. Birch trees can also be similar to Oak having extruding branches.

Here are several examples of differing trees. This one is tall and thin like Spruce.

This one is shorter and broader like an Oak tree.

Heres one that looks more like Acacia.

Add details with quartz blocks where it needs rounded, this doesn't really apply to the thin variant.

Example 1:

Example 2:

Birch trees have airy clusters of leaves around each branch.