I am going to recommend the textbook I use for my upper-level undergraduate courses in Mathematical Probability & Statistics:
Hogg, R.V., Tanis, E.A., & Zimmerman, D.L. (2020). Probability and Statistical Inference (10th ed.). Hoboken, NJ: Pearson. (ISBN 0-13-518939-X)

As a textbook, I like this because it is written in a format that many undergraduates can read and learn from using just the text itself.

If you feel comfortable enough with the probability theory introduced, you can start in chapters 5 or 6 to focus on the foundations of mathematical statistics.

This is (by far) not the only text available...but I hope this helps.