How to get Pre-doctoral opportunities?

I hold a master's degree in computational physics from India. Based on the feedback I received on my funded PhD applications, I believe I need to gain further research experience . I am therefore exploring pre-doctoral and research assistant positions in the Netherlands, Belgium, Finland, Sweden, and Switzerland.

I know how to approach professors through cold emails and how to apply for advertised PhD openings, but I am unsure how to find paid research assistants or pre-doctoral opportunities. Could someone explain the process for finding and applying to such positions?

My research interests include DFT, 2D materials, strongly correlated materials, and light-matter interaction.