For college, I know your intended major plays a big role as to what you want to get accepted in with. Is Economics an easy major to get into? What about Global Relations? What about going undeclared? Would it be more beneficial to go in undeclared to get into Economics (if it’s hard to get in)?
It depends on the college. Most colleges and universities do not admit by major - they just admit you the college, or whatever college/school houses the undergraduate economics major - and so getting in with an economics major is no easier or harder than getting in with any other major.
Even for lots of universities that do require you to apply to specific majors, economics is often not one of them (often it’s engineering and/or the sciences).
For the colleges that do require you to apply to economics, how difficult it is will really vary - it’ll depend on how many other students want to major in economics and how many resources (and therefore slots) they have for economics majors. Getting in at a school where there are only 10 slots each year for econ majors, but only 20 students are interested, will be more difficult than getting in somewhere there are 100 slots, but 1000 people interested.
At schools where you do have to apply specifically to economics, it might not be a good idea to go in undeclared and then attempt to transfer to economics later. There’s always the significant chance that you are unable to transfer into that major, and then what will you do? I mean, if you already really love the school for other reasons, it might be a great choice - you could just major in something else. But if you attended that school mostly for the economics major (which I wouldn’t recommend), you might be unhappy.
Same goes for global relations/international relations, with the additional note that you usually don’t have to apply separately for that except for maybe schools with really strong programs in that field.
It depends on the college. It seems that most colleges admit by school, but whether they also admit by major within a school is less clear. Applying for an economics major within a university’s Arts & Sciences (aka liberal arts) school may be slightly less competitive than applying for an economics concentration within a university’s undergrad business school. For many universities, the particular major within their Arts & Sciences school won’t make a big difference for admission (a common exception is computer science). My personal guess: econ within A&S is unlikely to more competitive than other majors within A&S and probably not difficult to transfer into from another major. If you think you want econ, then apply for econ.
Why make this difficult for yourself? Unless you for whatever reason must limit your college choices to schools of a certain type, you can apply to colleges at which your tentative academic interests would be only lightly considered, if at all, for purposes of admission, and at which all majors and most courses would be open to you upon attendance.