Welcome to the forum.
There are two ways (places really) to do dimensions, either in modelspace or paperspace.
I don't use ANNOTATIVE dims, will stay on thread topic with ASSOCIATIVE dimensions.
You will need to set up a single dimstyle which displays in the units of measure, and with the precision which you usually require.
There are lots of choices to be made in setting up a new dimstyle, you would typically start from the STANDARD dimstyle,
enter NEW, give it a new name and change or adjust whatever needs changing.
Dimension styles, once created are typically included in one's default template, in the interests of ease of use and conformity.
For now let's just assume we will use the STANDARD, or an existing dimstyle.
If you use associative dimensions then you really will be good to go.
Turn on your ENDPOINT, QUADRANT and perhaps your MIDPOINT OSNAPS.
Don't turn on NODES, as this will likely screw up the associativity of your dimensions.
I prefer doing dimensions and annotation in paperspace. Your viewports should be set up as you want them, scaled and LOCKED before you start doing your dimensions..
Start doing your dimensions, following standard dimensioning practices.
Be sure that the start and end points of each dimension are SNAPPED to a detail or feature of your drawing, as you are ATTACHING them to the geometry which the lines represent.
This is why I said to turn off your NODES, which are the defining points of your dimension lines.
You do not want your ASSOCIATIVE dimensions associated to other dimensions, but rather to the geometry which they define.
When done in this way you needn't concern yourself with the various scales which might be represented and used in your viewports.
The dimensions are all 1:1 based on the geometry to which they are ASSOCIATED (or attached).
If later the length of a beam, or the location of some holes are changed in modelspace the ASSOCIATED dimensions will reflect those changes in real time in your paperspace layout.
If there are NEW elements, holes or details added you will need to make sure you document those, in the same way as previously described.
Dimensioning in modelspace would be done in the same way, just in a different location.
All of these ASSOCIATIVE dimensions and dimension text will display correctly in your paperspace, as well as in your modelspace,
because their display is driven by the text height and other settings which are spelled out in the definition of the DIMSTYLE.
BE SURE TO CHECK in your OPTIONS > USER PREFERENCES, that the box MAKE NEW DIMENSIONS ASSOCIATIVE is checked.
There are two schools of thought on where text, dimensions & drawing sheets should be placed.
The company for which, or with whom you are working likely has standards to which you may be expected to comply.
If not, or you are working for yourself, or are the one making such calls, then do it how you feel it works best for the
kind of work which you usually do. Try it both ways first, make an informed decision.
ANNOTATIVE dimensions are a whole different animal, and I defer to other forum members better informed than I, on their best use, set up and practices.