Here are some general staples:
General Bioinformatics knowledge: https://www.biostarhandbook.com/
The paid handbooks were fantastic and well worth the price imo. Coding problems for bioinformatics: https://rosalind.info/problems/locations/
Learning techniques involving bulkRNAseq and scRNAseq: https://diytranscriptomics.com/
Also check out these threads: https://www.reddit.com/r/bioinformatics/comments/1bm3r9g/scr... https://www.reddit.com/r/bioinformatics/comments/191ykr/reso... https://www.reddit.com/r/bioinformatics/comments/12puflj/plo...