Swiss people were heard saying Guten Abend as "Gütte Oppa" in one part of the country. How is the Swiss-German spelling for this actually, and what are the various ways the different Swiss dialects say Guten Morgen and Guten Abend, etc?
Not really an answer since I'm by no means an expert, but I did find some relevant information. First, despite there being several YouTube channels purporting to teach "Swiss German", the German spoken in Switzerland is really a collection of related dialects. So not even a native Swiss would necessarily know all the variations possible in a simple expression. Wikipedia lists about two dozen main dialects and says there are many more sub-dialects within these groupings. The point is there is really no such thing as the Swiss-German word for something, you'd have to specify whether you're talking about Bern, Basel, Zürich or some other location. This video from the Easy German channel compares five different Swiss dialects.
I'm getting "Gueten Oobe" from a Speaksli channel video, and I mentioned it above because it seemed closest to what you were trying to spell out in the question. Note that the "gueten" is not a typo; the Swiss have a "ue" diphthong different from "ü" (not a diphthong). Apparently the Swiss tend to devoice their b's, d's, and g's a lot, even by German standards. Also, at least in the dialect the Speaksli channel covered, the "e" sound is very open. So "Oobe" sounds very much like "opa" to an English speaker. The Speaksli video uses native Swiss speakers, but it doesn't mention which dialect they're using, which seems very unhelpful. In the Easy German video one speaker, in St.-Galler-Deutsch uses "Guetä obig". Another, in Walliserdeutch, says "Güetn Abund". Another, in Berndeutsch, says "Guete Abe". A fourth speaker, in Churerdeutch, says "Guata obig". (One of the speakers doesn't say "good evening" in any dialect.)
Swiss German is primarily a spoken language; everyone learns Standard German in school so people are able to communicate with each other, and that's used as the written language. This means there is no standard spelling and everything is written phonetically. So not only do you have different dialects, you might see different spellings of the same word in a single dialect. It also means that it's difficult to find reference materials on Swiss German, so unless you happen to find a native of the region where a particular dialect is used, it will probably be difficult to get much information on it.
I'm wondering what inspired the question though. If you're planning a trip to Switzerland and attempting to learn Swiss German to prepare, then you should probably make sure to learn the correct dialect for the region you're visiting. I highly recommend the Easy German video to see what you're dealing with in terms of the differences between dialects. In any case, I think learning Standard German would be more useful; as I mentioned above, everyone in Switzerland learns Standard German in school and it's spoken in a large area of Central Europe, not just a single Canton.