When you buy an apartment, legally you do not buy "your own walls" but an apartment right. The law describes it as a share in the property included in the division, with the right to exclusive use of certain parts of the building that are intended to be used as a separate unit (article 5:106(4) BW). The share can also include exclusive use of a piece of land, such as a garden.
Apartment rights come into existence through a notarial deed of division that is registered in the public registers (article 5:109 BW). After that, an apartment right can be sold, divided and mortgaged on its own (article 5:117(1) BW).
The shares are equal unless the deed of division sets a different ratio (article 5:113(1) BW). That ratio, the fraction (breukdeel), often also determines your share of the costs.
See also: Apartment owner, Deed of division (splitsingsakte), Fraction (breukdeel), Private section (privé-gedeelte), Exclusive right of use
Legal basis: Boek 5 BW, art. 5:106, 5:109, 5:113, 5:117