What do internet techs do during network upgrades

Whenever Xfinity is doing "network improvements" in my area, it genuinely takes at least 3 to 4 consecutive days where (supposedly) a technican goes out somewhere and the internet goes down early in the morning, the ETA gets pushed back several times, before it eventually comes back up in the evening and my app tells me that "we're sorry but we couldn't finish today, expect another outage tomorrow."

At this point I need to know from the perspective of somebody who actually does this for a living what is happening. Does a routine upgrade really need to take almost an entire working week to finish? Are the technicians just the hands while the brains are remoting in or coordinating with them off-site? If it's something routine and pre-planned like a network upgrade, why are the expected ETAs so far off from how long they actually take?