Is it as competitive to get into as people say? Also, if you do get in is there a higher chance that you accepted to work with them in the future? Thanks
1. Year 10 work experience in Big 4 isn't competitive per se but rather I don't think they generally offer work experience in most of the firms due to the complexities involved in maintaining confidentiality/etc. There are always exceptions to the rule but I can't remember seeing it in the last 5 or so years. There's various other pathways to get paid/unpaid experience in these firms though but usually once you're in uni.
2. Marginally higher chance if you happened to impress upon them somehow that you were super well suited culturally to the firm... but realistically all the other factors which go into the recruitment process (eg extra curriculars, marks, work experience) are going to have far more weighting when they're considering cadetships, internships or graduate positions. You may be lucky enough to impress a manager so much that they're willing to be a reference in the future, but by the time you're applying for a role (even a cadetship), statistically speaking, there's a good chance they won't be there anymore.
The above isn't meant to sound cynical but rather just to highlight work experience big 4 doesn't mean much - sure it shows you have an interest/dedication to the career path which could be seen in a positive light for a future role, but I wouldn't stress too much if you don't get it. If anything all it will do directly is lead to the question of how you got work experience in the big 4 since it's not usually offered.