How to find a pattern in lines and append something to those lines?

https://unix.stackexchange.com/questions/543451/vi-finding-a-pattern-in-lines-and-add-at-the-end-of-those-lines

How to delete all the lines which contain a pattern in vim?

https://vim.fandom.com/wiki/Delete_all_lines_containing_a_pattern

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The ex command g is very useful for acting on lines that match a pattern. You can use it with the d command, to delete all lines that contain a particular pattern, or all lines that do not contain a pattern.

For example, to delete all lines containing "profile" (remove the /d to show the lines that the command will delete):

:g/profile/d

More complex patterns can be used, such as deleting all lines that are empty or that contain only whitespace:

:g/^\s*$/d

To delete all lines that do not contain a pattern, use g!, like this command to delete all lines that are not comment lines in a Vim script:

:g!/^\s*"/d

Note that g! is equivalent to v, so you could also do the above with:

:v/^\s*"/d

The next example shows use of \| ("or") to delete all lines except those that contain "error" or "warn" or "fail" (:help pattern):

:v/error\|warn\|fail/d