Popups vs inline forms for user input in a webapp

I'm not a UI/UX person. I'm building a webapp. I'm struggling to design my UIs for user input.

For example:

Some webapps (e.g. Slack, Asana, MS Teams, MS SharePoint) show a modal popup (with a form) almost every time you need to enter info.

However, other webapps (e.g. Github) show forms inline, in the page itself - this is harder to design, but the benefit is the form "flows" neatly with the main content, and may be more intuitive and less complex/confusing to the user.

From what I can tell, a popup is used when:

• you're lazy... popup design is easy, and no tinkering needed with the underlying page
• the amount of info to be collected is large
• the info is not relevant to the page, so it makes no sense to to embed the user input stuff into the page (so collect the info then hide it)
• are there other scenarios?

But popups are making it harder for me to make it responsive and look good on mobile.

Is there general advice in this area? Is one preferred to another given certain circumstances?

The problem is for those examples above - they took a design decision to do things one way or another across the entire site. They must have spent millions researching these topics, so I'd just like to copy the best practice.