The three pieces
Your registrar is the company that holds your domain name, the address people type to reach you. If the domain lapses, the website, the email, and your organization's name online all go dark at once, which makes this the account that matters most.
Your host is the company whose servers the website runs on. Think of it as the building the site lives in. When the site is down but email still arrives, the host is usually the one to call.
Your platform is the software the site is built in, such as WordPress, Squarespace, Wix, or Shopify. It decides who can edit the site, what it costs to run, and what it would take to leave.
Most people who inherit a website believe these are one company. They are usually three. Knowing which is which is the difference between calling the right support line during an outage and spending an hour on hold with the wrong one.
Do this
Go to lookup.icann.org and enter your domain. Write down the registrar name and the expiration date.
Go to builtwith.com and enter your domain. Write down the platform and, if it shows one, the hosting provider.
Open the organization's main inbox and search for the registrar's name and the host's name. Note which email address receives each company's notices. If it belongs to someone who no longer works there, mark it.
Put everything on one page: registrar, expiration date, host, platform, and the email address each account is registered to. Date the page.
Done looks like one dated page with three company names, one expiration date, and the email address on each account.
What to expect
Three things tend to turn up. First, the expiration date is closer than you would like for at least one domain; that is normal, and next week covers the renewal defenses that fix it. Second, at least one of the three accounts is probably registered to a personal email address, often a former employee's. Do not fix it today. Today is for knowing. Third, the lookup may show "REDACTED FOR PRIVACY" where the owner's name should be. That is privacy protection at work, not a problem, and the registrar and expiration date still appear.
That one page is the first row of the inventory you will build next, and it is the first thing your successor would need. File it where someone other than you could find it.
Go deeper
Two episodes of The Web Managers Podcast cover this ground in full. Episode 4: Domains, Your Most Important Digital Property separates the registrar, the host, and the DNS provider, and walks through what actually happens when a domain expires. Episode 6: Choosing Your Platform Without Regret includes a Tool Minute on BuiltWith, the same ninety-second lookup you just ran.