Some things I have recently been trying to understand:
the progress on cutoff for mixing times of Markov chains presented in Justin Salez's survey
strong convergence via Ramon van Handel's survey (shorter version for ICM)
concentration of measure and high-dimensional probability. I enjoyed:
Roman Vershynin's High-Dimensional Probability
Ramon van Handel's lecture notes on Probability in High Dimension
A. Bandeira, A. Singer, and T. Strohmer's Mathematics of Data Science
M. Ledoux's survey The Concentration of Measure Phenomenon
M. Talagrand's Upper and Lower Bounds for Stochastic Processes
Various ML stuff (with the knoledge that everything changes fast):
Bulgarian researchers in probability (with similar sites) that I know of are E. Dimitrov, I. Hartarsky, and L. Lichev.
Additionally, you can explore S. Apostolov's blog, which covers a wide range of intriguing mathematical subjects.
My master's thesis was going through Cutoff on all Ramanujan Graphs by E. Lubetzky and Y. Peres for which I clarified the proofs and found a non-trivial mistake (in Proposition 6, p<2). I was extremely lucky to be supervised by J. Salez.
I have written notes for the entry exam for the PhD program in FMI and for the exam after the first year and second year in it.
Around 2020, I was often recommending the online course Financial Markets and the specialization Deep Learning. I think the first is still educational and fun. For some basic understanding of gpt-like models, Karphaty's nanochat, Stanford's Language Modeling from Scratch, and some of the deeplearning.ai courses were useful to me.