Hi Sara. This is only a part answer to your question, but any tool of Moodle can be re-used and shared, either via import (if on the same Moodle platform), or backup (if between Moodle platforms). With the backup approach, this generates a .MBZ for just that activity, and an MBZ is just a modified Zip file
It then comes down to the tool that is best fit-for-purpose. The kicker in your question is that it is "For assessment", and this can mean different things to different folk, eg:
Formative assessment - an activity to challenge the student's learning
Good Moodle tools - Discussion, quiz, lesson, H5P, OU Blog, OU Wiki, student contributions to Glossary or database, etc.
Summative assessment - an assessment that contributes to their overall grade
(Depending on your sector, these assessments may need to stand up to an audit)
Good Moodle tools - assignment, quiz, graded discussion forum
It is worth knowing that any of these tools can include content in their design. For instance in a Quiz, you can use an "Description" question to include a short bit of content, or a video the student watches, then answers subsequent questions.
The Moodle lessons tool allows you to have content and activities sequenced together in the same activity ..... but lessons present some design issues, IMHO