A binary search tree (BST) is a binary tree with one rule: for every node, everything in its left subtree is smaller, and everything in its right subtree is bigger. That rule turns searching into a walk from the root: go left if the target is smaller, right if bigger:

class Node:
    def __init__(self, value):
        self.value = value
        self.left = None
        self.right = None

def insert(node, value):
    if node is None:
        return Node(value)
    if value < node.value:
        node.left = insert(node.left, value)
    elif value > node.value:
        node.right = insert(node.right, value)
    return node

def contains(node, value):
    while node is not None:
        if value == node.value:
            return True
        node = node.left if value < node.value else node.right
    return False

root = None
for v in [50, 30, 70, 20, 40, 60, 80]:
    root = insert(root, v)
print(contains(root, 60), contains(root, 65))

insert walks down the same way and attaches the new value where the walk falls off the tree. Duplicates are simply ignored here.