They say whatever furthers their personal objectives.

What they say being a lie or not is only for them relevant if lying now would later negativelly impact them (for example, because certain people won't trust them).

I think that what both Trump and Elon have figured out is that some people just think lies are normal and "everybody does it" so they don't really lose the trust of those because there wasn't any to begin with, true believers can be told whatever and they'll never stop believing and losing the trust of some of those of the rest who aren't super rich as them doesn't make much difference.

As far as I can tell that's pretty much the general rule for why and when people lie.

The difference between the likes of Elon and Trump and normal people is that normal people naturally feel guilt and disconfort when trying to deceive others and, unless they've trained themselves to stop that, that's reflected in their body language when they do so, thus other people might subconsciously detect it and feel that they're not to be trusted, whilst people like Trump and Elon can tell a lie looking you in the eye and look the same as if they were telling you that "the sky is blue".