I don't really get this part. Mind elaborating?

Lets take a camera lens as an example.

When a look the the schematic diagram of a camera lens, u will see a big lens at the front aka objective lens, follow by many other smaller ones in the middle and the rear. It is states a #of elements in #of groups. Example 11 elements in 6 groups.

If u rid of all the internal lens except for the objective lens, the objective lens will become dependent on the diameter of the lens (light gathering) and focal length, effectively becoming a telescope lens

What the internal lens does is to correct for aberrations arising from the main objective lens, flatten the field, zooming in zoom lens, etc.

A biconvex lens will bend light inwards, shortening the light path, thus the focal ratio will become smaller. Known as 'focal reducer'.

Example:

80mm diameter lens, 480mm focal length, f ratio is 6

with a convex lens, my focal length is shortened

80mm diameter lens, 400mm focal length, f ratio is 5

A biconcave will bend light outwards, increasing the light path, thus the focal ratio will become larger.

Taking the above example, 80mm diameter lens, 960mm, f ratio is 12

A tele-extender multiplies the focal length. 2x tele-extender multiplies by 2 times.

Known as 'barlow' in telescope terms

Hope u understand