I happen to store mine in Evernote. I have a “notebook” for each client, with projects and other details under that.
A single note holds all their server-related access such as hosting company, control panel, FTP, database, IPs, nameservers, software versions and more. Typically the customer has me setup email accounts for them so I put that here too, as well as all access to the website backend itself, whether Wordpress or some other CMS.
The reason I keep all this is that I organize it and send it back to the customer so they have a detailed record of all their accounts too.
I believe it would be difficult to do this in an app, because the data is so arbitrary. If they host with one hosting company or another, I’m storing very different details. An FTP might be just a domain with user/pass, but there may be multiple accounts. Same with email, or Wordpress users. Maybe they don’t use Wordpress but some other tool? If they have Godaddy, they might have a pin number, but other companies don’t have a pin, but maybe they have question/answer challenges I need to record.
DNS data may be handled at the host, or with a DNS provider. Or maybe they use a proxy like Cloudflare.
I feel like, if this were going to be wrapped up in a software, then all the fields have to be arbitrary, but also allow grouping and subgrouping and building up custom templates. For example a “user” is not always just a username and password. Maybe it’s a username, and email, a displayname, and a password. But not all users belong under one heading, there are users for the hosting company, for FTP, for email accounts, and for the software like Wordpress. So multiple users are linked together under a single heading, but each “user” under different headings might have slightly different details.
I believe this is why if you ask this question, most people will respond by saying they type it all up in a big text box, because you can type whatever you want. There is no real standard way to store all this, and as soon as you try to develop “standard fields”, somebody will need them altered, or something added to it.
Good luck tho!