Are you even clicking the links I'm providing and reading those?

"I am questioning whether the mega backdoor contributions"

Let's drop "Backdoor" because that process is after any funds have been put (contributed) into any account. Backdoor is part of conversion, not part of contribution. A Trad IRA contribution limit might be $7,000; that same Trad IRA account might have $150,000 in it and I can convert all $157,000 to Roth, and there was no Roth contribution in this paragraph, because those converted funds are not contribution to the Roth. They already existed. Even though there is a $7,000 contribution limit, there is no $ limit on the conversion.

"are considered elective deferrals"

So... what happens later (backdoor or not) is moot to the issue here. You are stating this backwards.

Start here: Deferrals of an employee's pay, at the employee's election, are contributed to the employer plan by the employer. These are elective deferrals by definition. What the employee does with them later, such as retires and takes distributions, rolls into a different employers' plan or out of plan to an individually held account, and/or converted in-plan to Roth, is not going to change the nature of the contribution. There may or may not be employer matching; it depends on the plan's provisions (not on the IRS). Congress and thereby, the IRS, control which provisions are allowable, not which provisions are offered.

"and subject to the $23k limit?"

The IRS maintains limits for how much you can contribute to a qualified retirement plan:

https://www.investopedia.com/terms/e/electivedeferralcontribution.asp

https://www.investopedia.com/terms/1/401kplan.asp

https://www.investopedia.com/retirement/401k-contribution-limits/

"If yes, how can employees contribute 23k + after tax contributions, which I know many of my clients do."

That's what I pointed out. It is possible those after-tax contributions are not allowed. And, your clients can be getting bad advice from brokers. You need more details, to know the answer to what or how and if it is allowed or not. Employees of what entity type, putting into what plan type, what is the after-tax component provision? Are they making a catchup contribution because of age? Are they maxing out their 401(k), then making a nondeductible Trad IRA post-tax contribution? You stated mega-backdoor Roth, which typically is 401(k) conversion to Roth 401(k). This?

https://www.schwab.com/learn/story/should-you-consider-roth-401k

"Choosing a Roth 401(k) or a traditional 401(k) might not be an either-or decision. If your employer offers both, you can contribute to a Roth 401(k) and a traditional 401(k). However, keep in mind that your annual contribution limit would apply across both accounts. For example, you can’t contribute the 2023 salary deferral limit of $22,500 ($30,000 if you’re age 50 or older) to each 401(k). Instead, you must divide the total amount between accounts—for instance, putting $11,000 in a traditional 401(k) and $11,500 in the Roth 401(k). The same goes for your total annual contribution amount ($66,000, or $73,500 if you’re age 50 or older), which includes employer matches."

You have to know the plan type (401(k), safe harbor 401(k), Solo 401(k), SEP IRA, SIMPLE IRA, etc), its provisions, the relationship (employee, shareholder, sole proprietor, highly compensated employee?) and perhaps you are describing people who have an employment position (paid via W2) as well as Sched C, like Greg there.

SECURE Act 2.0 allows the employer match to be Roth directly, at the employer's discretion. Or, "the employee" is putting the employer's share into Roth 401(k) as a match, or as the nonelective share (but they call it theirs because it's their company)? Or, maybe these employees are contributing their elective deferral as Roth and letting the employer match be Traditional 401(k) and then converting via mega-backdoor?

You can't use the words "plan" or "account" or even "contribution" out of context. You also can't use the word "employee" out of context. Just like you are trying to refer to multiple 401(k) sidestepping the issue of "different employer" vs "one employee job + a self-employed sidegig."

Perhaps this will help; it's the nonelective part:

https://www.investopedia.com/terms/n/non-electivecontribution.asp

There are a lot of variables. It's a matrix.