This is an interesting question with much written in magazines and online concerning the question of how chess is played ‘AC’ as opposed to ‘BC.’ That is Before and After the rise of the machines.
Before continuing I must comment on “…Bobby kinda represented the type of chess played before technology started changing how chess was studied and played.”
This is interesting because all who became World Campion brought a different Chess philosophy to the board. It is possible to posit that a WCC will influence style as much in Chess as a POTUS will influence style in the US, if not the world. I am thinking of how JFK influenced style as opposed to Ike. But to say, “…Bobby kinda represented the type of chess played before technology…” could be a stretch. Remember, Bobby’s contemporaries included players with drastically differing styles. For example, Mikail Tal and Ulf Andersson. In addition, I should mention Tigran Petrosian and Bent Larsen. One could posit Anatoly Karpov is a better example of “…the type of chess played before technology…” since he was the last WC before the rise of the machines. Kasparov was the WC during the rise of the machine, as everyone knows, and that does not mean only Chess players.
As for the question, “…about the changes of chess play/theory since the 1970’s”… The first thought that popped into my brain upon reading these words was an article I read by a GM concerning the difference in endgame play sans adjournments. He wrote because of adjournments there was a DEEP study of endgames, which is lacking today. The next thought was an article I read online some years ago with deep endgame analysis of some of the younger players butchering endgames, which led the author to conclude that the players of former generations were better when it came to the endgame. He also wrote that with the advent of ever increasing shorter time controls things would not improve…
The other thought caused me to go back and seek a comment from Chessbase. It concerns the game between Richard Rapport and WC Magnus Carlsen played in the eight round of the recently concluded Tata Steel tournament. Magnus played 23…Ne5, given a ?! by the annotator, Alex Lenderman. After Rapport played 24 Bg2 (! Lindy) Magnus replied 24…Rc8. Lenderman gives 24…Ned7, writing, " The funny thing is stockfish (sic) thinks (No, “it” does not WMB) going back ned7 (that should be (Ned7) is the best move…"
Having studied many games of the TCEC championships over the past couple of decades one thing I have noted is that the “engines” will not hesitate to “advance to the rear.” I am reminded of the great Capablanca, who once moved his Rook from a1 to c1, then returned it to the original square the next move. After the game the journalists questioned him about his strategy, which led to a long endgame victory for Capa. He answered, “I realized moving the Rook to c1 was a bad move and returned it to where it belonged. If I had not made the move to c1 I would have won in 40 moves, not 80.”
It has been written that the WC, Magnus Carlsen, is best at “grinding.” I like to think Magnus looks at it like, “OK, it is a small advantage, but it is an ADVANTAGE!” I know Bobby thought of it that way…Then how to explain Magnus trading one of his two Bishops versus Karjakin in the match for the WC, which led to a draw, in lieu of keeping it on the board while continuing to torture Sergei? Was it fatigue, or did the WC become frustrated at the “irresistible force” emanating from his opponent? In some respects WC Magnus Carlsen best typifies the change brought to Chess by the “engines.” Still, I prefer the games of Rapport and Nakamura, just as I preferred the style of Tal as opposed to that of Karpov. This is not to say that the style of Karpov is not as good as that of Tal, just different. In some respects the play of Karpov and Petrosian can be consider deeper positionally and was closer to the way the machines “play.”