What Are the Recommended Learning Material for Ssis

.Net vs SSIS: What should SSIS be used for?

good question.

if the amount of data transfer huge? are you processing multiple data files and need transactions (both at file system level and database level)? are you dealing with multiple data sources at different locations (for eg ftp, local file system, database)?

if answers to above are yes then go ahead with ssis. basically .net is cool with small data import / export jobs, but when you have anything more complex, ssis is a definite winner

the other thing which i look at is - is it worth writing .net code when everything is available inside ssis. (dont mistake me - i love coding) however, anything you code, you need to maintain :-)

Which systems memory will be used by ssis package?

Yes. A package runs on the same computer as the program that launches it. Even when a program loads a package that is stored remotely on another server, the package runs on the local computer.
If by server resources you mean server CPU, you cant. Is like using resources of any other computer on the network. Of course, if you have an OleDBSource that runs a select on SQL server, the CPU that "runs" the select will be the one on SQL Server, obviously, but once the resultset is retrieved, it is handled by the computer where the package is running.
Like any other development method. If you have a class on a C# project being developed by 3 developer, how do you do it? You can have each developer working on the same file and merge the changes, after all a package is a xml file, but is more complicated. I wouldn't recommend. I've been on situations where more than one developer worked on the same package but not at the exact same time.

SSIS - best practices for connection managers -- compose out of parameters?

SSIS, starting from version 2012, has SSIS Catalog DB. You can create all your 50 packages in one Project, and all these packages share the same Project Connection Managers.

Then you deploy this Project into the SSIS Catalog; the Project automatically exposes Connection Manager parameters with CM prefix. The CM parameters are parts of the Connection Manager definition.
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In the SSIS Catalog you can create so called Environments. In the Environment you define variables with name and datatype, and store its value.

Then - the most interesting part - you can associate the Environment and the uploaded Project. This allows you to bind project parameter with environment variable.
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At Package Execution - you have to specify which Environment to use when specifying Connection Strings. Yes, you can have several Environments in the Catalog, and choose when starting Package.

Cool, isn't it?

Moreover, passwords are stored encrypted, so none can copy it. Values of these Environment Variables can be configured by support engineers who has no knowledge of SSIS packages.

More Info on SSIS Catalog and Environments from MS Docs.

Managing security for an SSIS package accessing another SQL server - best practice

The best practice for running SSIS packages by using the SQL Server Agent is that SQL Server Agent is run by a domain account (actually this is the best practice for all SQL Server services, you should never use the defaults in production environments) and create a credential and a proxy account for running the SSIS packages.

From the security point of view, you could create the connection to SQL2 in the SSIS-package and provide the connection with a hard coded user and password. To protect the credentials, you can use a Package protection level that suits you (password protection might be the easiest): https://learn.microsoft.com/en-us/sql/integration-services/security/access-control-for-sensitive-data-in-packages?view=sql-server-ver15#protection-levels

SSIS Importing Excel Data

Why inserting data into one Excel worksheet??

Inserting data into one Excel worksheet or even one workbook (Excel file) is not a good practice at all, you have to think in another way, you can create a new Excel file each time new data comes and save historical data in another repository or directory (if you need to). Or as @TabAlleman suggested if you can use flat files, it is more recommended since reading data from Excel is more difficult. But also make sure that you will not store all data in one flat file.