You are correct to look at the Iterate Feature Classes tool. One of its outputs is the file name. You can then use a python expression of function (Calculate Value tool) to generate the corresponding path and then do the join. Join Field may be a better approach than add join / copy features, depending on what you're doing. Here's an example of the calculation to generate the second file name,
# this is an example what you get from the iterator Name output variable
# (data used below in variable reference %Name%)
"T1"
# here is an example of what expression you could
# use with the Calculate Value tool
# this would evaluate to :"T__1_ExportFeatures1"
"T__1_ExportFeatures{}".format("%Name%".replace("T",""))
You then use the output variable from Calculate Value for tools used down the chain, like Join Field.
A tip: if needed, use preconditions, for example, make sure the iterator "Name" variable is set as a precondition to the Calculate Value to make sure you are working with the value from the current iteration.
Hope this is helpful to you!
