Important points to note

[ishanijerin1]

Cost
To use web links, you will need to prepare a server and your own domain.
Security
Creating an association between your app and your website is more secure than a custom URL scheme, but Apple's official documentation states the following:
Warning
Universal links offer a potential attack vector into guinea email address your app, so make sure to validate all URL parameters and discard any malformed URLs. In addition, limit the available actions to those that don't risk the user's data. For example, don't allow universal links to directly delete content or access sensitive information about the user. When testing your URL-handling code, make sure your test cases include improperly formatted URLs.

(↓Rough Japanese translation)

Universal Links potentially provide attack vectors, so be sure to validate all URL parameters, discard malformed URLs, and limit the actions available in universal links to avoid putting user data at risk.

Restriction example
Prevent content from being deleted directly via universal link
Prevent access to sensitive information about users
App Links (Android)
It is a mechanism provided by Android that is equivalent to iOS Universal Links.
The basic concept is almost the same, so I will skip it this time.
※Reference:

Handling Android App Links (official documentation)
[Qiita] About Android AppLinks (Comparing App Links and Universal Links)
summary
Even if you say "deep link" in a word, if you don't understand which technology is used to implement it and how it works with the other system, there is a possibility that there will be a discrepancy in understanding what you can and can't do.
When you start development, you also need to select the technology to use based on the requirements and specifications.
I've never implemented deep linking from scratch before, but what I learned this time has deepened my understanding!