Q1: Does intended major make it easier or more difficult to get into a college?
A1: The answer depends on the college. You need to look into each college specifically.
Some colleges admit only to the school, and all majors nominally have similar selectivity, although admissions readers grading subjective aspects or doing holistic reading may be affected in unpredictable ways.
Others admit by division (e.g. College of Arts and Sciences, College of Engineering, College of Business) or major, because some divisions or majors may be enrolled to full capacity.
Some colleges admit by major in some divisions but not others.
Some majors at some colleges admit only to a pre-major status, where you will need to earn high enough college grades and/or compete for admission to get into the major later. In some cases, pre-major status is no different from general undeclared status; in other cases, it is different in that students not in the pre-major will find it more difficult to enter the major later.
Among colleges that admit by division or major, some will only admit to the specific division or major, or reject entirely. Others consider second choice majors, or may admit to an undeclared status.
Q2: What if I apply to a less selective major or undeclared (or are admitted to a college but not my intended major) and then change my major later to a more selective major?
A2: If a major is more selective when applying to admission to the college due to the major being at full capacity, then it is likely to be selective for enrolled students trying to change into it later. If you try this “back door” route, you may find that doing so requires a high college GPA and/or competitive admission process. You may want to web search “[college] change major [desired major]” to see what you can find.