Blogs!!!!!! for me Brent Ozar's is top
PodCasts - Data Exposed is good but is more for Azure than T-SQL
Most importantly - Practice - Get a free copy of SQL 2019 Developer and try (using one of the sample db's or better still make your own db). Set up some sample problems and try to solve them with the best query you can.
For example, write some code that outputs the numbers 1 to 100, but if the number is dividable by 3 write fizz, if by 5 then pop, if by both fizz pop.