If you are considering to enter the field of systems and circuits neuroscience, the following resources would be extremely helpful in preparing you for the experimental and analytical methods that are necessary.
Approaches to study neural circuits
Online videos and material from a highly-relevant class organized by Luke Sjulson.
Neural data science
Online course on the basics of analysis of neuroscience data by Philipp Berens.
Basic theory of extracellular electrophysiology
Online material for the fundamentals of electrophysiology
Last Black Box course
Fundamentals of electronics, signals, and neuroscience
Neuromatch academy
This is an excellent online resource for computational neuroscience.
Check especially all the pre-reqs refreshers.
Computational psychiatry course material
Check the preparation material and lecture recordings.
Methods in computational neuroscience (course videos)
See individual subfolders for each year. Valuable lectures.
Complexity explorer
A number of courses on dynamical systems and complexity
Non-linear dynamics and chaos
A course by Steven Strogatz
Cajal course in Computational Neuroscience (online lectures)