Software for Preparing Editions
Text editors
MS Word / Libre office (obviously)
LaTeX
Rather difficult to learn
Very powerful
Online courses
Software
Oxygen XML (multiplatform, paid 💵)
XML Copy Editor (free)
jEdit (with a couple of plugins, multiplatform, free)
EditTEI
not tested
Online XML editor
not tested
does not seem to be suitable for TEI XML
Projects
Connecting images and TEI XML