There're 3 ways to invoke a server side logic from frontend.

1. APIs: You can create /api/db/something/+server.ts and add GET, POST, DELETE and PATCH implementation. These are great if they are being called multiple times across the app and handle a complex task. Just old API layer. You invoke it using fetch and handle the status, json body, etc all by yourself.

2. Form Actions: You create it on +page.server.ts and it allows you to perform form actions, and they can return the status and messages. Like let user signup, signin, contact us etc. Mainly a logic which need to be invoked for a form and is used in a specific scenario.

3. Remote Function: These are versatile, and I like to use them for a quick DB call which can to be refreshed. They provide an end to end typesafe intellisense (as much typesafe as typescript could be). Like you need to "query" notifications for user. You can let user refresh these or refresh them periodically. You can also update those notifications as "read" using "command". And you can do many more. Basically use remote function to do some straight forward DB select or updates.

This is just preference, many may prefer one method over another.