What is a 401k plan in some contracting agency that is 0% to 100% contribution and limited to 25 funds?

The IRS limits the amount of your salary you can defer into a 401(k). In 2021, that limit is $19,500 but it changes year-to-year. If you are over 50, you can make an additional $6,500 catch-up contribution. The employer may not set an upper limit on the percentage you can defer but you will be limited in the total amount you can defer.

401(k) plans may allow you to roll over funds to an IRA while you are still employed but they need not. If your plan doesn't allow in-service rollovers, you'd be stuck with whatever funds the plan makes available so long as you are employed there. Once you leave that employer, you'd be able to roll over the 401(k) to an IRA with a wider array of investment options.

Each 401(k) plan (or 403(b)) is an individually-tailored plan developed by collaboration between the employer offering the plan and the financial institution providing it. Some big-picture items are defined by law and/or IRS rules, but the rest will vary from plan-to-plan.

Generally applicable to 401(k):

• Employee voluntary contribution (total of pre-tax plus Roth, if part of plan) is limited. There is a "catch-up" provision that allows older employees (over age 50) to contribute an additional amount. Currently these limits are $19,500 per year, with an additional $6,500 allowed for employees over 50. These limits tend to increase over time (but not necessarily every year); the IRS typically announces the new limits for each year ahead of the calendar year.
• Total contributions (all employee contributions, including those not covered in the first bullet, plus all employer contributions) are capped as well. Currently, this limit is $57,000 per year (plus the $6,500 catch-up for employees over 50)

Plan-specific variations:

• Matching - how much an employer contributes (if anything) to a plan is up to the individual employer.
• Roth option - some plans allow Roth contributions, and some do not
• Additional after-tax non-Roth contributions - sometimes allowed, can possibly be used for "mega backdoor Roth IRA" if plan allows
• Loans - whether a plan allows loans, and what rules are in place for taking an repaying a loan, can vary
• In-service withdrawals - whether you are allowed to withdraw money from the plan while still employed by the employer offering the plan
• In-service conversion - whether you are allowed to convert pre-tax money in your 401(k) to your Roth 401(k) while still employed by the employer offering the plan (this is a taxable event; the converted money counts as income)
• Investment options - Money in a 401(k) can only be invested in funds that are offered as part of the plan. Some plans have a very wide variety of choices (as you saw with BrokerageLink), while others have very few. Note that once you move the money to another account (roll to an IRA or another 401(k)), you can invest it in anything you like (potentially subject to similar limitations of the new 401(k) plan)
• Fees - The financial institution may charge you a fee directly for the plan. The fee may be collected as part of the investments. Depends on the specifics of the plan.