Usually, if you use installinstallmacos.py, you’ll get the already-released installers:

# ProductID Version Build Post Date Title
1 001-15219 10.15.5 19F2200 2020-06-15 macOS Catalina
2 001-04366 10.15.4 19E2269 2020-05-04 macOS Catalina
3 061-86291 10.15.3 19D2064 2020-03-23 macOS Catalina
4 041-91758 10.13.6 17G66 2019-10-19 macOS High Sierra
5 001-57224 10.15.7 19H4 2020-10-27 macOS Catalina
6 061-26589 10.14.6 18G103 2019-10-14 macOS Mojave
7 001-51042 10.15.7 19H2 2020-09-24 macOS Catalina
8 001-36735 10.15.6 19G2006 2020-08-06 macOS Catalina
9 041-88800 10.14.4 18E2034 2019-10-23 macOS Mojave
10 041-90855 10.13.5 17F66a 2019-10-23 Install macOS High Sierra Beta
11 061-26578 10.14.5 18F2059 2019-10-14 macOS Mojave
12 001-36801 10.15.6 19G2021 2020-08-12 macOS Catalina

If you’re in the Apple Beta Software Program (technically, you don’t have to be, but you may want to be to make sure you’re in compliance with Apple terms of use), you can run

sudo installinstallmacos.py --seedprogram CustomerSeed --ignore-cache

And that should get you full installers for beta releases as well:

# ProductID Version Build Post Date Title
1 061-77704 10.15.4 19E242d 2020-02-26 macOS Catalina Beta
2 001-15219 10.15.5 19F2200 2020-06-15 macOS Catalina
3 001-06847 10.15.6 19G60d 2020-06-30 macOS Catalina Beta
4 001-58883 11.0.1 20B5012d 2020-10-29 macOS Big Sur Beta
5 001-04366 10.15.4 19E2269 2020-05-04 macOS Catalina
6 001-36801 10.15.6 19G2021 2020-08-12 macOS Catalina
7 061-86291 10.15.3 19D2064 2020-03-23 macOS Catalina
8 041-91758 10.13.6 17G66 2019-10-19 macOS High Sierra
9 001-57224 10.15.7 19H4 2020-10-27 macOS Catalina
10 061-26589 10.14.6 18G103 2019-10-14 macOS Mojave
11 001-51042 10.15.7 19H2 2020-09-24 macOS Catalina
12 001-36735 10.15.6 19G2006 2020-08-06 macOS Catalina
13 041-88800 10.14.4 18E2034 2019-10-23 macOS Mojave
14 041-90855 10.13.5 17F66a 2019-10-23 Install macOS High Sierra Beta
15 061-26578 10.14.5 18F2059 2019-10-14 macOS Mojave
16 061-44345 10.15.2 19C39d 2019-11-15 macOS Catalina Beta