What is SN1 and SN2 Reaction?

I found this analogy of cat and comfy chair from this website. It's really helpful in making me understand the concept of SN1 and SN2 reaction.

SN1 reaction happens just like the cat in the first row. The yellow cat wants to sit in the comfy chair, but it needs to wait until the grey cat leaves the comfy chair. So, once it's empty, the yellow cat occupies the comfy chair. So, the SN1 reaction is the substitution where the nucleophilic (the yellow cat) is not too strong so it can not get rid of the leaving group easily, it requires to wait until the leaving group leaves and the carbocation formed. This reaction is also called as unimolecular because the reaction is only depending on one thing that is the substrate only where the carbocation formed. The tertiary alkyl halide, for example, is the most possible/the fastest than the secondary and primary one. Because the tertiary one is having the most stable carbocation. Mostly the product of the substitution is a racemic or the mixture of retention and inversion.

While the SN2 reaction occurs like the cat in the second row. The yellow cat is strong enough to get rid of the grey cat, so the grey cat is forced to leave the comfy chair. So, the SN2 reaction is the substitution where the nucleophilic is strong enough so it can get rid of the leaving group easily. This reaction is depending on two things: the substrate and the nucleophilic, so that's why this reaction is bimolecular. It depends on the type of the substrate, the primary one is easier than the secondary and tertiary. Because in tertiary one, the steric hindrance is big enough to hinder the nucleophilic to get rid of the leaving group, while it's so easy with the primary one where is no steric hindrance. As just mentioned earlier, not only the substrates, the nucleophilic itself is also affecting the SN2 reaction. The more concentrated the nucleophilic it is, the faster the reaction goes.