What's the default units of scale?

Is the default scale good for building big objects like houses? Say a wall has 3 meters, should I just input “300” when creating the surface or should I change some parameters?

In scene settings you can change to metric where 1 blender unit would then show up as 1m. Change the scale value to change to cm or mm etc

If you just use blender units, just use what is the most appropriate. Best not to use very very small of very very big values

If you have a 3m wall you could use 1 BU = 1m. If you want to then add an ant crawling on a brick in your wall in fine detail maybe that is not the most appropriate scale for the whole scene if 90% of your further modelling will be in fractions of a mm

For physics simulations 1BU is treated as 1m

Treat 1BU as 1 meter unless you have a reason not to, like if you are modeling a building that is too large to fit on the grid, or something really small that becomes hard to rotate around because of that. If you need to scale up or down, do so in powers of 10 so that you can calculate the correct size quickly by moving the decimal place. Remember that measurements will not be accurate if you have an object scale other than X1,Y1,Z1 so always Apply Scale unless you have a reason not to, like if the original size will need to be reverted to at some point.