Think of a double-sided doorway: whether you enter from the left or right side, there is a designated entry point on each side. Similarly, light entering a lens from either direction creates a focus on both sides.

2 Surfaces = 2 Foci ($F_1$ & $F_2$)

When light rays parallel to the principal axis fall on a lens, refraction occurs at both curved/flat surfaces. In a convex lens, these rays converge at a single point on the opposite side, while in a concave lens, they appear to diverge from a single point on the same side. Since light rays can travel from left to right or right to left, a lens has two symmetric focal points equidistantly placed from its optical centre ($O$).

Remember that while a lens has 2 principal foci ($F_1$ and $F_2$) and 2 centers of curvature ($C_1$ and $C_2$), a spherical mirror has only 1 focus and 1 center of curvature.