Understanding B-Trees The Data Structure Behind Modern Databases by Spanning Tree

Where are the values stored in a B+ tree?
In a B+ tree, the values are stored in the leaves.

What happens to a value in a leaf when a node splits in a B+ tree?
In a B+ tree, when a node splits the value stays in one leaf or the other.

In real world usage, such as in SQLite, do nodes have a fixed number of keys?
No, in real world usage, such as in SQLite, nodes don't have a fixed number of keys.

Do keys and / or values have a fixed or variable size?
In real world usage, such as SQLite, keys and / or values have a highly variable size.

What are the sizes of nodes?
The sizes of nodes are fixed-size disk pages.

When do you split a node?
You split a node when its size in bytes overflows a page.