There are no published/proposed abort modes for ascent. We can speculate that some incidents can be survived by letting Starship tumble free and light it's own engines to burn to a low orbit or land depending on altitude and velocity, but that capability has not been proposed by SpaceX.
Landing redundancy is provided by carrying 7 deep throttling, sea level capable engines on the current edition - meaning a redundant number of engines can start the landing burn at low throttle levels, increasing throttle if engines drop out.
But even that is speculative based on the current design. SpaceX has not discussed abort modes other than mentioning that airlines do not have escape capsules or parachutes, and stating that they are aiming for that level of reliability.
From the first IAC presentation, a vague response from Elon about abort abilities and their impracticality with this platform:
Oh launch abort, the spacecraft itself is capable of aborting from the
booster, the erm… Launch abort on the spaceship itself is kinda
pointless, if you’re on Mars you’re taking off or you’re not taking
off. You know, parachutes don’t work too well and [you can’t have]
some standard abort system, and just how do you abort 100 people it’s
just not feasible, the key is to make the spaceship itself extremely
safe and reliable, and have redundancy in the engines, high safety
margins and have [it be] well tested. Much like a commercial airliner.
Like they don’t give you parachutes.