Aren't all the 38s a remainder from Arriva days?
I vaguely having a recollection of there being a 36/37(?)/38 and 39 bus.
There was also additional 38s provided by Dickson's.
Trying to do this from memory! Arriva used the old Clydeside numbers - same route from Glasgow to Paisley with variations round Johnstone.
36 Glasgow - Kilbarchan via Johnstone Centre
38 Glasgow - Spateston via Johnstone Castle and Cochrane Castle
39 Glasgow - Spateston via Johnstone Centre.
There was a 30 which was Spateston - Cochrane Castle via Johnstone Centre too which is long gone, Key Coaches run part of it now. I don't remember a 37.
Dickson's had a 38, mainly minibuses between Glasgow and Paisley. McGill's bought this and expanded the service. Once McGill's bought Arriva they renumbered all of the Paisley Road West routes to 38/38A.
The 38s beyond Paisley all go to Kilbarchan or Spateston via Johnstone Centre, using the old 36 and 39 routes. The 38A is a peak time variation which skips Johnstone Centre and uses the 'old 38 route' via Johnstone Castle.
Johnstone Castle is served by the 20 bus during the day but at nights and Sundays when the 20 is finished the 38 to Spateston will double back from Quarrleton Road to serve. Not overly simple.
With frequencies like that, I can fully understand why Arriva were keen to off-load their Scottish operation!

None of those areas could come anywhere near justifying those service levels.
Most of the core routes across Renfrewshire seem to be at around the correct level, or at least they were pre-Covid.
With vehicle capacity at levels around the 18-20 mark, there are a number of services with journeys that regularly come close (or exceed) those capacities. That is something that all the main operator’s (incl.McGill’s) are experiencing, hence why there are a variety of measures being adopted to keep people moving.
It was worse in the 1990s- most routes had some form of competition and some of it was silly stuff. A lot of the independent operators are gone and McGill's are left. Running old minibuses with 5 folk on it might have worked in the 90s, but not today.
Much of it is based on economic reasons. Paisley have suffered like many Scottish towns, as industry closes and shopping slumps and moves out of town. This reduces demand and has caused removal of many bus routes- Paisley doesn't have links with places a few miles away like Houston any more, Johnstone and Linwood aren't linked well either with supported services focused on maintaining coverage not usefulness.
McGill's need to be weary of cutting too much- it can harm marginal links and if services are not frequent enough they will lose passengers. I think the 17 is an example of this- removed links west-east of Paisley and frequency drop means it's not turn up and go. They have to walk a tightrope on that.
Equally, multiple operators with different tickets on corridors leads to over provision and you think a degree of regulation could assist in fixing things for the benefit of operators and passengers. First running the 9 to Paisley just isn't needed, when McGill's could cover, as Covid showed. The problem is single operator tickets.