What's the best way to study middlegames?

Modern Chess Strategy by Pachman Or Simple Chess by Stean are both Excellent books on this topic. 

There are a large number of factors that can go into making a middlegame plan. The challenge is you have to understand each one in order to know how to utilize it. 

The list (not exhaustive) would include-

1. Time (not clock time, but tempi) 

2. Space

3. Force (the material count)

4. Pawn Structure (weak pawns, doubled pawns, isolated pawns, pawn couples, backward pawns, Pawn chains, passed pawns, outside passed pawn, etc.)

5. Open Files

6. Half-Open Files

7. Knight versus Bishop inequality

8. Outposts

9. Color strengths & Weaknesses (meaning white or dark square strategy)

10. Minority Attacks

11. Initiative

12. Piece Co-ordination

13. Relative Position of Kings (same side, opposite sides) 

14. Rooks doubled on the 7th or 8th ranks

15. The bishop pair

16. Good versus Bad Bishop

Thanks! Yeah the quarantine definitely freed up my schedule for just chess.

John Bartholomew's channel is a terrific, I like it a lot. I'd also recommend Hanging Pawns and Chessfactor. Both channels explain concepts very clearly and I'm learning a lot.

As far as taking the jump from 1300 to 1400 I noticed I'm starting to visualize the board better and figuring out what my opponent is doing. Maybe work on calculation? Can't say for sure but certainly can't hurt

By using short and long term piece activity. At lower levels, the pawn structures give a hint. However, to really understand how and why it's necessary to go into piece and pawn coordination (Kasparov's Quality of Position). A couple of classic examples are Nezhmetdinov-Chernikov (1962) and Reshevsky-Petrosian (1953).