I am 76 years old and started traveling overseas in 1981 when I took a job with the Army Corps of Engineers in Saudi Arabia. Prior to that time, I had been to Mexico and Canada.
I always planned touring back then, but in the 1980s way fewer tourists visited Europe, much less the rest of the World, you didn't have to reserve anything, you just when were you wanted and saw it.
I later took a job with the Army (civilian) in Augsburg, Germany for four years and did a lot of site seeing on our own, only rarely taking tours of a few days or a week. While living in Germany, we just hopped in the car for a weekend or perhaps took a few days off and had 8-10 days in a row to tour. We stayed in inexpensive BNBs or small hotels. We had less money to spend, but still did quite a lot.
After I retired in 2010, we started taking 2 or 3 overseas trips a year, visiting Europe, Asia, South America, Australia/NZ. Then our travel usually included a cruise with a planned self guided land trip (not a group tour). Since we have gotten older, we have gravitated toward group tours where a tour company takes care of lodging, guides, transportation, admissions, etc. I have visited 83 countries outside the USA. Life is good.
More directly answering the bullets you cited above, when we did trips largely on our own, this is what we did.
1) I would research places to see in specific countries or areas (like central Europe, or Iberia, British Isles or eastern Mediterranean. Largely using guidebooks prior to the internet, but by the 21st Century, largely with the internet. I used sites like TripAdvisor.com "things to do" in a specific city.
2) I would plan a schedule to visit specific places, including lodging, transportation, admissions, etc. For example, in doing a four week drive tour of South Wales and England in 2017, I planned landing at Heathrow, renting a car, driving to Bath, staying there for 3 nights, I made decisions on what we are to see and we generally stuck to those plans. I even planned using internet travel tools how long it would take to drive from say Bath to Cardiff.
3) We would not wander aimlessly and hope we found something nice. I learned in 1988 planning a trip to Luxembourg for a long weekend that not making plans for lodging was not wise, when the small town we planned to stay had not lodgings due to an annual festival.
4) We have gone back and visited some places again, but much of our travels that has not been the case. I have been to Venice 6 times, Florence twice and Rome twice. Athens 3 times, but Istanbul once. I visited Egypt three times, the last time with my wife, because she had not been there and I was there over 40 years ago.