But is it truly random or are there constraints in how computers are
built that makes them not truly random number generators?
The CPU of a common computer operates deterministic, given the same input, it will always calculate the same output. So by itself it is not able to come up with a random number.
What you typically see are pseudo random generators (PRNGs) which during their deterministic run can go through a series of numbers with very large cycles and have other favourable properties, which seem to be random and can be used as approximation to random numbers.
E.g. you could involve the system time into the start value of the PRNG to make the generated number seem more random, but if you had a second exact copy of the first system with the same system time, it would generate the same numbers as the first system.
However modern computers are not isolated machines, but have sensors and network connections. These sources can be used to "harvest" entropy.
An example for this is the system behind https://www.random.org/ which measures atmospheric noise and provides it via the internet.
If there is true randomness in nature is open to debate.
In the realm of classical mechanics one assumes deterministic behaviour, but one might loose track fast due to huge number of interaction partners and sometimes extreme sensitivity to initial conditions and amplifications of errors. That is kind of practical randomness.
In the realm of quantumn mechanics one has the interesting middle ground that individual events can seem to occur at random (e.g. radioactive decay), but nonetheless the distribution of probabilities develops according to strict laws, like Schrödinger's wave equation.
How would one figure out the difference?
A simple test is using the random numbers as coordinates of points in 2D or 3D space and plot them.
The expectation is that the view plane or volume is filled up uniformly.
However if pattern form, this is an indication of regularity in the sequence.
E.g. see https://www.random.org/analysis/
Or see this interesting article: Strange Attractors and TCP/IP Sequence Number Analysis.