There are a few different marking systems for SMT resistors and some resistors are not marked at all.
Probably the most common is a system with three or four digits that is similar to resistor color codes but with digits instead of numbers, the last digit is the multiplier and the preceding digits are the significant figures so for example "150" is 15Ω.
For values where the multiplier would be less than (or sometimes equal to) zero, the letter R is used as a decimal point instead of using a multiplier. So "1R0" is 1.0Ω, "R10" would be 0.1Ω. https://electronics.stackexchange.com/a/565417/88614 says that for even smaller values the letter m is sometimes used for milliohms, but I haven't come across this personally.
A single 0 on it's own generally represents a zero ohm link.
There is also a code system called EIA-96 that uses two numbers follwed by a letter to represent values from E96 (which would require four digits to represent with a traditional numeric code). The number is an index into E96 with "01" representing a value of 100 and "96" representing a value of "976" the letters then represent the multiplier, I won't copy the full set of letters here, but they can be found at https://eepower.com/resistor-guide/resistor-standards-and-codes/resistor-smd-code/#
I have vague recollections of another alphanumeric system but with the letter at the start rather than the end but I can't remember the details and google isn't turning it up right now.
The size is sometimes driven by power requirement and sometimes by ease of assembly. In the more common sizes you can usually find all values from E24 and E96 with no problem. In more esoteric sizes there may be a more limited range.
Unfortunately there are two different systems for size codes, an imperial system where the size code approximately represents the size in hundredths of an inch and a metric one where the code represents the size in tenths of a millimeter. Unfortunately unless you already have an idea what size resistor is involved you can't tell just by looking at a code which system it is. https://eepower.com/resistor-guide/resistor-standards-and-codes/resistor-sizes-and-packages/ has the dimensions for some common sizes (though don't assume because a size is not in those lists that it doesn't exist).