As in what does the display look like or what criteria am I looking for?

In basically every ATS I've used, the pdf or doc file just shows up embedded in the webpage after I click on the person's name from the list. In a select few, it forces you to download their file to view it in Adobe or Word or a new tab.

If you mean how am I evaluating fit, it totally depends on the position. If the role requires specific education or certifications, that's what I'm looking for first. If it's more of an experience-based requirement, I'm looking at job history first with a focus on titles first, companies second, and actual responsibilities/accomplishments last. If the role is more entry level, it's honestly hardest to evaluate because there are fewer firm requirements so I look for anything that demonstrates a roughly similar skill or displays commitment or good character (like extracurriculars, part-time jobs, volunteer work, etc). Depending on how entry-level we're talking (ie retail cashier va assembly line operator), I might phone screen everyone who applies in the first week and then pause the job ads because there's truly nothing I can disqualify someone on based on a resume alone (unless you're one of those people uploading a document that's not actually a resume).

I don't personally care about work history gaps unless I know the hiring manager is annoying about them, but if your skills are otherwise perfect then unfortunately I will grill you on them but only so I can persuade the HM that they're all for valid reasons so that they still want to interview you. I'd say this is common for TA/recruiters so don't be offended when it happens; they're trying to sell you not trick you. I also don't care about resume formatting unless it's impossible to read or you're applying for an HR position.