do guitars need to be altered to hold thicker gauge strings?

Assuming that the guitar was perfectly set up for a particular string gauge (say 0.009" - 0.042") and you used slightly heavier strings (say 0.010" - 0.046") then you would probably need to make slight changes to the intonation and maybe the action if you wanted it to play perfectly. In practice though, the guitar might be fine if you adjusted nothing and with an acoustic guitar you can't adjust the intonation or action easily anyway.

In the above example, if you were changing to strings that were a lot heavier (like changing from a set starting 0,009" to a set starting 0.012") then you are almost certain to need to adjust the intonation, the action and maybe even the neck relief. It's also possible that the nut and pickup height might need adjustment too.

The time to make any adjustments on a guitar is when it needs it, regardless of whether you've changed anything or not. Over the years, I've made quite dramatic changes in terms of string gauge or tuning to some guitars and needed to adjust nothing. At other times I've needed to make adjustments to a guitar where nothing's been changed for years.

Assuming you mean gauges and types of string that are normally sold for a particular type of guitar, and not something totally off the wall, some adjustment may be required for optimal playability. I wouldn't really call it alteration. Occasionally nut slots might need enlarging but it is rare for them to made without enough clearance for strings in a normal gauge range. And your tech might need to adjust the neck curvature and/or the action slightly.

This type of adjustment is called a "setup".