PMS & CHANNEL MANAGER RECOMMENDATIONS BELOW
--- 1-3 rooms:
1. http://SYMPL.cm ($)
2. http://Hosts.Guesty.com ($$$)

--- 4-10 rooms try:
1.http://Hospitable.com ($$)
2.http://LittleHotelier.com ($$)
3.http://Tokeet.com ($$)

--- 11+ rooms
1.http://Guesty.com ($$$)
2. http://CloudBeds.com ($$$)

This is an update on the old 2018 video:
How to sync your Calendar between AirBnB and Booking.com
no need to use iCal or google calendar if you only need these two, for more than two I suggest using an external calendar... if you want the best speed use a channel manager.