First, make sure the slate of glass is straight, this can be checked with e.g. a metal ruler on its side against the glass surface. If the glass isn't straight (which should be per the production process of glass, but there have been reported bad glass beds) you never get a perfect level over the whole bed. Also make sure the glass sits on a clean heated bed plate (no debris between the heater and the glass).
Second, level the bed, start with powering the printer. You need to sequentially do the following as adjusting the one corner (screw), affects the other corners (continue this until the bed is level and the one corner doesn't affect the other corners anymore):
- Home the machine,
- heat up the bed and nozzle to e.g. PLA printing temperatures,
- move the nozzle close to a corner (a different one than the previous corner),
- put a piece of plain printing paper on the bed,
- lower the nozzle to Z=0,
- adjust the screw in that corner until the piece of paper can be dragged under the nozzle with a slight resistance,
- repeat by starting with homing the printer.
After several rounds of leveling and having a level/straight bed to begin with, you should have a leveled bed that has been leveled against the printers' X-axis.
Now, when printing something it should be level, the only thing that might not be correct is the distance between the nozzle and the bed. E.g. some users prefer a larger distance between bed and nozzle when printing PETG (not my personal experience, but a generally accepted truth). This distance can be tuned without having to re-level your bed; you could if you want to use a thicker or thinner paper, but you can easily change the Z=0 by redefining the Z=0 level at e.g. 0.10 mm height if the nozzle to close. Some slicers even allow you to add an offset (e.g. the "Z Offset Setting" plugin in Ultimaker Cura from developer "FieldOfView").
When you have dialed in the distance also correctly, you should get perfect prints.
Do note that a common issue with these over-constraint "cantilever" printer designs is that by powering a single side, the opposite needs to follow exactly, that is a challenge with that many parts. My preference is using dual lead screws, preferably driven by a timing belt for Prusa type printers.
Addressing the BLTouch part in your question; before you wander in the world of automatic bed leveling (AB) you should first master getting a level bed, or fix the X-axis rollers on Z beams. For ABL you also need to level your bed first else you get non-square prints. The roller solution is one of the major drawbacks of these printers, you need to make sure the X-axis (aluminum extrusion bar) stays level (or better trammed) in relation to the bed level, loose rollers should be properly tensioned.