Easy way: be sure all the files are in the same directory, open the first yxdb file with a Input Tool, change the connection string using metacharacters:
I want to open all the yxdb file in C:\User\Dataset. I start opening the file C:\User\Dataset\First_file.yxdb - then change the string C:\User\Dataset\*.yxdb (Any yxdb file, no matter the file name)
Classic way: You have to combine 2 tools, Directory tools (or any other way to get the full path list of the yxdb files you want to connect to) + Dynamic Input tool.
The Directory Tool gives you a list of all the files inside a folder - and optionally subfolders too. After that you can connect a Dynamic Input tool. To configure the tool properly you'll have to:
open one of your yxdb files as template (it only works if the several files have the same structure, number of fields, data type and header names)
Select the source of list of files (the field Full Path created by the directory tool)
Choose the action Change Entire File Path
The Dynamic Input tool basically opens the file you set as template and then sequentially opens all the other files listed in the directory, changing the entire file path of the first one used as template, and unions them (that's why we need a template).