Submissions are moderated, but depending on the field, anything reasonable-looking may be accepted. For example, there is more than one "proof" of the Riemann Hypothesis in the arXiv. But the moderation system certainly keeps out spam.

Edit: Here's a proof of the Riemann hypothesis submitted just last week: https://arxiv.org/pdf/2209.01890.pdf You can have a little fun exploring "proofs" of famous theorems on the arXiv over the years.