Does “a point you choose” include any movable surface?

I read somewhere that spells like Darkness and Silence, where the effect of the spell spreads from a point I choose within range, can be cast on the surface of a moveable item. But after extensive googling, I couldn’t find confirmation on it anymore. These are the relevant parts of the spell descriptions:

Darkness
Magical darkness spreads from a point you choose within range to fill a 15-foot radius sphere for the duration. …

Silence
For the duration, no sound can be created within or pass through a 20-foot-radius sphere centered on a point you choose within range. …

So theoretically, does this mean that a Warlock with the Devil’s Sight incantation can cast Darkness on their armor and "carry" it around with him? Giving him semi-permanent (Concentration) advantage to targets without Devil’s Sight/Truesight?

Another example would be someone casting Silence on an enemy mage, rendering them unable to cast spells with Verbal components also semi-permanently (Concentration)?