Well, "algorithm" isn't a perfectly well defined term, and to be precise about randomness we sometimes say "randomized algorithm" if an algorithm uses an RNG. You can also extend the notion of Turing machine to include a randomization feature if you need that for something. I would say this whole issue is usually not important. It shows up mostly in some niche areas.

I would expect the CT thesis would gloss over the issue a little bit, and at the end of the day allow randomized computations as being realizeable.

Can I ask what book you are using? The ones I know of are now pretty old, I guess.

I don't know of instances where RNG's can make an uncomputable function computable. I think maybe that never happens. It is an open research question (called P=BPP) whether an RNG can allow solving a problem efficiently where the only non-randomized solutions are inefficient (i.e. a PRNG won't work, you need a real RNG).

The studies about computability and Turing machines etc. started in the 1920s, before there were computers, so the subject area was mathematical logic rather than the then-nonexistent "computer science". These days CS is mostly about specific algorithms, and even complexity theory (a niche area in theoretical CS, which in turn is a niche area in CS) is mostly about the computable, and stratifying computable functions into layers according to how many computations it takes to solve them.

If you're interested in the more "out there" questions of what can be computed at all, you might benefit from studying some logic (I mean the kind taught in math departments). Computability theory is imho mostly a topic in logic. It overlaps CS, but only somewhat. I think any theory-oriented programmer should study logic at least a little though, say enough to understand how predicate logic works. Unfortunately I don't know of a good book to suggest, but maybe someone else here does. There are good Wikipedia articles in the topic, but I find it useful to have something to read in linear order.