Stability of Halogenoalkanes

In forming Alcohols or other substitution:

Is it correct that
i) tertiary carbocation is the most stable as it has 3 alkyl groups surrounding the C-atom which has lost an electron and became a cation
ii) but the tertiary halogenoalkane which is the basis for this reaction is actually the least stable and thus most reactive?

I.e. there is both most and least stability in this structure:
i) most in terms of the cation that is surrounded by the alkyl groups and
ii) least in terms of the overall molecule that is reacting fastest? And the reason that tertiary halogenoalkanes are least stable and thus able to react so fast is due to the cation that can be formed only as it is being stabilised by the alkyl groups. So effectively, the stability and instability are kind of linked.

i) "tertiary carbocation is the most stable as it has 3 alkyl groups surrounding the C-atom which has lost an electron and became a cation"
Yes

ii) "but the tertiary halogenoalkane which is the basis for this reaction is actually the least stable and thus most reactive?"
Sure, if you are considering S_N1 reactions only
For S_N2 reactions tertiary alkyl halides are least reactive.

Thank you but I thought SN1 is there only route for tertiary halogenoalkanes (or at least preferred) due to the fact that an intermediate carbonation can be formed?

Yes this is true. SN1 is not a viable reaction route due to steric effects. Hence tertiary do not react via Sn2. [which is why I said tertiary is least reactive towards Sn2, in fact there is no reaction (or very little reaction)]

If you look at the interactive I uploaded you will see that both mechanisms are "viable" but the rate at which they occur depends on the stucture of the haloalkane (and the solvent used).