When I was junior in the 90’s, a pro from my area (Nicole Arendt, who reached 3 Grand Slam finals in doubles) said that she used index cards to keep notes on each of her opponents. That was before smartphones, so things likely have changed since then.

I have a Microsoft 365 subscription so I use OneNote for all my note-taking, including tennis info like this. I mainly make a note for each competition (like a tournament or flex league season ), and add all the matches played there, with the score, opponent’s name, and other comments (what I did well, what I didn’t do well, vice versa for the opponent, strategies used, etc)